What do you need?
I'll begin with the simplest part. There's a rather small chance that you by mistake buy a DVD with a soundtrack you can't play, but it is possible. It's a bigger chance that you'll be disappointed when a film doesn't utilize all the sound channels in your home cinema equipment.
To play the sound on DVD films you'll most often need just a normal TV (or PC with a sound card if you buy/ have a DVD-ROM) with mono or stereo speakers. But that's of course not recommended. One of the greatest advances with DVDs is the possibility to play Dolby Digital encoded sound, so the ultimate choice is an AC3 amplifier with Dolby Digital support and preferably also DTS. Dolby Surround Pro-Logic will also give you a very good cinema feeling (equals Dolby Stereo previously used in cinemas), even if the program lacks the separate subwoofer channel and only has one mono rear channel. I wouldn't bought Dolby Surround today. To utilize Dolby Digital optimal you'll need six speakers, two front (four is possible), one center, two rear and one subwoofer for the deepest bass frequencies. The same applies for DTS. Some people thinks DTS is better than Dolby Digital, but there are few DTS titles. More about DTS vs. DD later. You can connect the same number of speakers to a Dolby Surround amplifier, but the subwoofer has no seperate channel, and the two rear speakers are just to spread the sound more in the room. It's Dolby Digital and DTS that's basically used in cinemas today. Dolby Surround Pro-Logic is a «home-version» of the old Dolby Stereo system. Many films (and then also DVDs) are only encoded in this format.
Formats
Below is a summary of the various audio formats you're likely (and in some cases unlikely) to encounter on DVDs.
 Dolby Digital Mono
With Dolby Digital you'll just get sound in the center speaker. Not very normal fortunately. Used on older films (for instance the first release of The Terminator and low-budget releases) where they haven't wanted or haven't afforded to make a new soundtrack. It's of course also used on commentary tracks. Personally, I'd recommend to turn the effect on your amplifier off in this case, so you get sound in both your front speakers instead. Remember, you will have the same sound experience as in the cinema.
 Dolby Digital Stereo
With Dolby Digital you'll get sound just in the two front speakers. Not very normal. Used on some music DVDs and commentary tracks.
 Dolby Digital Surround 2.0/ Dolby Stereo
With Dolby Digital you'll get sound in all the speakers except the subwooferen. The background sounds will be in mono. Equals Dolby Surround (with the exception that you will get sound in the subwoofer with a Dolby Surround amplifier). Very common, and often the best choice for us with a Dolby Sorround Pro-Logic amplifier. The reason for 2.0 is that Dolby Surround really is encoded as a Stereo soundtrack and then the 4 channels are decoded by the amplifier. This allows TV channels to transmit in Dolby Surround. Often provided in addition to a Dolby Digital 5.1 track.
Dolby Digital 4.0
The same as Dolby Digital Surround 2.0.
 Dolby Digital 5.0
With Dolby Digital you'll get sound in all channels except the subwoofer. The background sounds separated, that is, different in the two rear speakers. Occurs rather often, often on Disney releses (for instance Tarzan), but the productions companies often receivers critic for this kind of soundtrack, because it doesn't utilize the subwoofer. An unexperienced sound technician can also happen to mix the sound wrong so the result is DD 5.0 instead of DD 5.1. A pity for those with a subwoofer of course.
 Dolby Digital 5.1
With Dolby Digital you'll get sepearate sound in all channels. Different sound in all speakers, that is. «Real» Dolby Digital, and what utilize the format best. As good as every new film has this soundtrack, often provided in addition to a Dolby Digital Surround track on the DVD. Be aware of that the DD Surround often is choses as standard, so you must manually select the DD 5.1 track if you have a Dolby Digital amplifier. In some cases, it can actually be better to playback the DD 5.1 track with Dolby Surround Pro-Logic equipment. It's just to test both and find out which one you prefer, but in most cases you'll loose some deep bass sounds. What is very bad is that some studios actually downmix the cinematic 5.1 soundtrack to DD 2.0 on some DVDs (for instance Mission to Mars here in Norway). So be careful too choose the correct version of a DVD. Check out the links page for sites that provide comparisons between DVD releases in various countries. This is of course also listed in my films section, where I've been able to find the information.
MPEG MultiChannel
MPEG was the sound system Philips, among others, wanted on DVD instead of Dolby Digital. They lost that fight, but it's released a few DVDs with MPEG sound, mostly in Europa as far as I know. There are MPEP soundtracks provided with the Swedish films Ögat and Spring För Livet. Scanbox' Bean has only MPEG. It's theoretically equal to Dolby Digital 5.1, and is also considered equal in quality in Europe, but practical experience shows mostly trouble with this sound system. It's also not used in any cinemas in the world.
 Dolby Digital 5.1 EX/ THX Surround EX
An new improved version of the Dolby Digital system, basically with a center rear channel in addition to the two existing rear speakers. The new sound information is mixed into the signals for the two existing speakers, so it's not actually 7 separated channels, and is because of that called DD 5.2. The home cinema version is refered to as THX Surround EX. You will need a new amplifier to take advantage of it, but can be played on a normal DD amplifier. Not very common yet of course, but provided on T2: The Ultimate Edition DVD and first used for Star Wars Episode I.
DTS
Warning! With Dolby Digital only you won't get any sound! To playback DTS you must have a DTS amplifier. The same number of channels as Dolby Digital 5.1, but is to some extent considered as «real» six-channel sound, probably because it utilize the subwoofer to a higher extent. The first film with DTS soundtrack was Jurassic Park, but many people was disappointed by the DTS DVD version. It's getting more and more common to publish DTS DVDs, but because this format is compressed only 1/4 of what Dolby Digital is, there's isn't space for many extras on those releases (Dante's Peak, Waterworld and many more). A Dolby Digital Surround track is often provided too.
DTS ES
An new improved version of the DTS soundtrack. You will need a new amplifier to take advantage of it, but can be played on a normal DTS amplifier. Not very common yet of course, but provided on T2: The Ultimate Edition DVD.
PCM
With Dolby Digital you'll get stereo sound in the front speakers. Normal uncompressed CD sound format. Usual on music releases, to get the best sound quality. In most cases I'd recommend to turn off the effect on your amplifier, especially if it's Dolby Surround Pro-Logic, to get a true stereo sound. Otherwise, you'll get sound from the center and rear speakers aswell.
Sony SDDS
This format is, as far as I know, only used in cinemas. DTS and Dolby Digital is considered better by most people.
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