A lot of people (myself included) have been having problems with sound in doom 3.
There are two problems:-
1) You cant select surround sound
If you have this issue make sure that both your Windows speaker settings and your Sound card driver applet are both set to the same setting. For example in Creatives drivers you must make sure that what is set in your Control Panel is the same as is set in Sounds and Audio Devices / Speaker Settings / Advanced. Also make sure that under the Audio tab of Sounds and Audio Devices, that your sound card is set as the Default device.
Unfortunately Doom 3 does not support true 4.1 either but there is an alternative:-
From Anandtech:
"For those out there with 4 speaker or 4.1 setups (or those who just want to hook two pairs of speakers up to their system to save a little on the cash flow, we tested audio with an old 4 channel system. We noticed that the center channel on our 5.1 system, in addition to playing noises that were directly in front of us, is used for radio communications, and audio played back via the PDA. But all is not lost for those without a center channel. Doom 3 seems to play all center channel data through the front left and right speakers as well. Not having a center channel doesn't impact our amazing surround sound gaming experience in any real way. Yes, the PDA, radio communication, and the like loose just a bit of distinction, but the real meat of the system is to be able to locate enemies via audio. This is just as amazing using 4 channel sound as it is with 6."
article here:
http://www.anandtech.com/guides/showdoc.aspx?i=2155&p=3
and secondly
2) After a short amount of time the sound starts to crackle and then cuts out or just cuts out completely when using 5.1
This is the problem i had - it is caused generally by an app you have running in the background - usually some sort of monitoring program like Motherboard Monitor or AtiTool.
In my case ATITool was causing my sound to cut out - i needed to disable the temerature monitor feature in order to get this to work - once i did this - the sound was restored and it dosent cut out.
Enjoy.