NeonBible said:
How would I achieve this without re-compressing the AC3?

If you’re talking about modifying the file itself, then you can’t. This sort of change requires re-encoding.

If you want to do it on playback, some people use ReClock. I’ve never used it myself and don’t intend to start. Based on what you’ve described, I’d probably change the frame-rate of the video too and re-encode the audio.

0.94 would be about right.

If you decide to re-encode, try this.

LoadPlugin("TimeStretch.dll")
WhateverSource()
TimeStretchPlugin(pitch=94)

I’m not sure I understand your reference to 8%.