I left all my slow motion parts silent because I couldn't be bothered messing with it.
I would still like to know how though.
This guide has some info:
http://www.halflife2.net/forums/showthread.php?t=87850
Here is copy paste of relevant section:
How to make audio slow motion:
* The theory of this is simple, so is the practice of it, but here it is: Exported audio is normal motion always. So to make it fit slow motion, slow it down as much as the slow motion is.
So if your movie has 30FPS as normal motion, and your slow motion is exported at 180FPS, which gives 1/3rd of normal motion, you need to make the audio three times as long (meaning you have to slow it down so it's three times as long. This is called resampling in a lot of audio editing software, cause that's what you do basically).
If all you have is Microsoft Sound Recorder (which I believe comes with all Windows versions and is found in the Accessories > Entertainment folder in your Start menu, or simply Windows Key + R and type in sndrec32 and hit Enter). Then your slow motion options is limited. Cause all MS Sound Recorder can do is make the sound half the speed or twice the speed, so you only get slow motion at half the speed of normal motion, or half of that again, 1/4th of normal motion and so on. I'm gonna assume you only have this software though as most of you will have that available.
So if you have slow motion footage exported with either 60, 120 or 240FPS (1/2 speed, 1/4 speed or 1/8 speed), then slow the original WAV file down either 1, 2 or 3 times to get the correct length and it should fit perfectly still. Just save it and then do the procedure as above with thise new resampled sound and you should have your high quality slow motion footage ready for editing.