You can do this (and a ton of different cool stuff) with RogueAmoeba's Audio Hijack Pro app, which lets you route audio from one app to another, optionally appl

You can do this (and a ton of different cool stuff) with RogueAmoeba's Audio Hijack Pro app, which lets you route audio from one app to another, optionally applying some filters to it. I sometimes use it to fix audio out-of-sync problems in certain videos. Edit: As commenter above noted, the audio routing functionality is also available with the free Soundflower app.