Running 32 bit Windows XP on Steam, with at least 2GB installed RAM? If so do this.
1: Locate the "FalloutNV.EXE" file. Normally found in "..Steam\SteamApps\common\fallout new vegas" directory.
2: Right click that file and select "Properties".
3: Go to the "Compatibility" tab.
4: Check box the "Run this program in Compatibility mode for.." option.
5: Select "Windows 98 / Windows ME" from the pull down menu.
That should sort out the sound lag. Well, at least it did for me.