Is there a way to modify audio playback rate?

I’m working on producing video clips of games and I really need to be able to run games slower so I can record them at a much higher framerate. I’ve managed to do that just fine by using speed hacks to slow the game down to a fraction of the normal speed, which is great and nearly there. The problem with this is that it doesn’t also have the effect of slowing down the rate that the corresponding audio plays at. e.g. I can record a gun firing, at 300 frames per second (by reducing the speed of the game to 20%, but the sound of the shot is not also stretched out, so when I play those 300 frames per second in real time afterwards (normal speed), it has the effect of the sound being extremely short and high-pitched. The sound I record with the video is useless.
I’ve spent hours and hours googling and searching through files and thinking, but I’m stuck. I even considered unpacking the .pck files, stretching out all of the audio files by a fixed amount (300%) and then repacking them, but I wouldn’t know where to start, and on top of that, I’ve discovered it’s probably illegal.

So if anyone knows if I can modify something so that the play speed of the ingame audio can be lined up with the play speed of the game, that would be awesome. I was really excited about this but I’m stuck at the moment. I’m not even sure where on the internet I should ask this, but I figured it probably has something to do with the game engine. The game is Dishonored, if that helps.

To sum up, If anyone has a way to make a game play its audio files really slow-mo like in action movies, you will have made my week.