Let me throw out some info about this question and cover a bit more concerning how UBCD4Win ( as the team ) may see the issues as well as the PE community as a whole can or does look at it in regards to the drivers themselves and not so much on the programs that are used for those or other purposes.
UBCD4Win's main focus is and has always been with diagnostics, repair and recovery as the main or primary goals. This happens to be what the original BartPE was developed for primarily and the PE community has extended it well beyond those goals to also include using PE for many other areas...
The BTS DriverPacks are by far the most complete set of drivers found anywhere in the Windows community so it was just natural that the PE community looked at them as the "holy grail" so to speak in this area. As the BTS team can attest too... its no small feat to combine all of the drivers into one set.
I think its obvious to most users why the mass storage drivers are probably the most important for diagnostics, repair and recovery purposes. While there can be no perfect solution since new drivers are introduced all of the time... the BTS driver packs certainly cover the majority and their only exceptions are limited by technical reasons such as the standard ide driver vs the raid ones... or simply because a new driver was introduced that hasn't yet been integrated.
Many neophite ( newbies ) users would think that the video drivers would be the next most important... that really isn't the case. Most video cards are also vesa compatible so even if a specific driver isn't available, they will still work at the higher resolutions. Then, those that may not be completely vesa compatible... are still VGA compatible so video is always covered even if basically.
Actually... the LAN drivers are just a bit more important than the video... With lan drivers a user can save their files to any server... whether its on their own network or a web server with FTP access. So for recovery purposes... this can be very important.
Now to address the question that started my long explaination and why a user may want sound drivers. ( Its about time eh... LOL )
Sound drivers are actually helpful in the diagnostics area for a PE build... I'm not talking about just being able to play your favorite music... but to be able to tell if the sound card is actually working. Of course the problem for this issue is still going to be the size of a build... to cover a large majority of sound drivers would also probably end up requiring a DVD instead of a CD... This would probably mean requiring a seperate PE build just to be able to test sound for most any system. This makes this area one of the last that will likely be addressed.