Audio - Decoding Acceleration - Unlike video acceleration, decoding acceleration does not infact speed up the decoding, but rather produces a better quality output. Most of these "accelerated" functions comes in the form of allowing 4 - Speaker discrete surround.
Audio - Decoding Quality - Software Players with "bad" sound decoding quality usually makes the sound-track sound soft and low in volume. A "good" quality means that the sound-track is clear, volume is loud, and the bass channel (LFE) is decoded properly. Currently, Cinemaster has the best quality, especially it's LFE decoding capabilities.
Audio - Decoding Speed - Audio Decoding speed is not very important as most sound card these days are more than enough for DVD decoding as long as they fit the minimum requirements. However, if you want the more "fancy" features such as 4 speaker discrete surround, you'll need to utilize a little bit more CPU power, and have a suitable sound card.
Audio - 4 Speaker discrete surround - Most DVDs are encoded in the Dolby Digital AC3 (5.1) sound system. This means that the surround sound information is encoded into the movie sound-track but it still allows a proper sound to be played on a standard 2 speaker system.
4 channels discrete surround has a front left/right, simulated center, rear left/right all at CD quality - all decoded by your sound card and some CPU power. Compared to Dolby Pro-logic, which only has a mono-rear channel and also Dolby Digital, which has an extra LFE (bass) channel, 4 speaker discrete's quality falls somewhere in between.
Audio - SPDIF Support - Sometimes you may wish to let an external amp/decoder to do the audio decoding for you, instead of your software decoder, to produce a better quality sound. To do this, you'll need a sound card that has digital output (in the form of SPDIF). The new range of SB-Live sounds cards have this feature. You'll also need software that supports this output. Currently, only very few software decoders supports this feature.
VOB Playback Support - VOB files are the native format of DVD movies. Most DVD movie discs will play if you insert them into your DVD-ROM drive and press play, no need to load up the individual .vob files. Some players, like the QI player, does not allow you to play these .vob files. Most other players do. Note : Normally, you cannot just play the .vob files straight off a commercial DVD disc by selecting "file-open...". This is because most DVDs have a kind of copy protection called CSS encryption. More information on CSS encryption is available in Chapter 6 - DVD-Video Basics Chapter.
Memory needs - Different software decoders have different video memory needs. While the superior quality Cinemaster decoder requires a hefty 4-6MB of free video memory, PowerDVD only requires 1-2MB - but at the expense of having poorer video quality.