The problem:
Single core machines are getting audio lag and certain sound effects are not playing at all (NPC's talking, shooting or reloading guns, etc.).
There's already a few possible fixes floating around, such as changing the priority of the game process to above normal or high, but doing that (in my case) desyncs audio and video, sometimes speeds up (or slows down) game, and makes the game unplayable by adding imput lag (where you press one key and you character keeps on walking in that direction).

The solution:
Download the silent hill homecoming patch v11 from here http://enbdev.com/download_en.htm. and put the vars_pc.cfg file in the data folder and the rest in the fallout new vegas folder where the .exe is. Doing that fixed my audio problems. Keep in mind that the ENB series of tweaks can lower performance a bit, so you might need to lower the settings to medium-high to get your game running smoothly. In my case it runs fine on maximum settings.

Solution was found here (credits to the original poster):
http://www.thenexusforums.com/index.php?/topic/248968-possible-sound-fix-and-character-creator-issue

Other:
I haven't tried this fix on any other builds since New Vegas runs fine on my other builds (after the patches).
I've heard of audio problems happening on dual core cpus, but my quad core systems didn't have this issue.
This fix might or might not solve audio problems on multi-core cpus.

This fix worked on this build:
Athlon 64 3500+ (2.2ghz)
2GB RAM
ATI Radeon 4850 (512mb)
Windows XP 32 bit (SP3)

(I have 2 better builds, but I wanted to get it working on all, since the old build could run FO3 on max settings.)
(Also I am not the only one at home who wants to play New Vegas...so I needed to get it working everywhere)

Yeah it does work, sorta. I cant use anti-aliasing with the fix.. and I would very much like to since it makes the grass and other objects ( transpercy samplign enabled ) look a lot better. If you know a workaround or how to edit the dll file or w/e to be able to sue aa, I would appreciate it. I would love a patch for the sound issues as soon as possible but I doubt they are even working on these things..

Does this work on the Steam version, by any chance?

Thanks, it worked like a charm.
Game runs smooth too.
I was about to toss the CE box out of the window.

I don't see why it wouldn't.

When I try this fix, my the FNV graphics settings read my GPU as a Nvidia Geforce 8600. My actual GPU is a ATI Raedeon X850 xt 512mb. Any ideas?

holy crap this fixed all my problems, i can now play the game in full screen without any sound lag, i had to run it in a window to get rid of like 90% of it before, frame rate as increased ten fold too, thank you!

My problem? No dialog sound and only music. Dialog interaction boxes also skipped through very quickly, to fast to read really. Digging through the forums it seemed a digital audio/codec problem. I have ffdshow installed through the Combined Community Codec Pack, this seems to be the problem. My solution?

Open the ffdshow audio decoder configuration.
Go to the "DirectShow control" page (select on the left side)
If nor already ticked, tick the box that says "Don't use ffdshow in:"
Click edit and add FalloutNV.exe as an exeption. You can find this file in your ProgramFiles(x86)/Steam/steamapps/FalloutNV directory

I've done the same for the ffdshow videodecoder but maybe that's not necessary.

Works like a charm for me. :wavey:

I must have missed to mention this. I use CCCP too and I read somewhere that it was necessary to add the FO:NV game process as an exception for both the video and audio decoder. I don't think it worked in my case but it apparently works for some people. Before applying the silent hill ENBSeries fix, nothing worked... even uninstalling CCCP and ffdshow. (I can't recall if I had the fallout .exe added in ffdshow audio/video decoder exception before I applied the silent hill enbseries patch 11, so if the patch doesn't work I advise doing the ffdshow exception...)

Awesome fix!
Cured a multitude of problems I was having involving audio and memory horrors.
Slows everything down slightly, but I'm running at 1920 by 1080 on high settings and haven't bothered tweaking anything yet.
Thanks bunches!

This post needs a bump, because there are multiple similar solutions to very different problems.

This is NOT the same solution as the one involving DirectX10 and Windows 7.

Of the various d3d9.dll solutions this is the only one that works for me and my ATI Radeon HD 3850 AGP card on a Windows XP system.