Large music files make asset importing slow / game large. Compression ideas?

We currently store all our audio assets (be they game music or sound effects) as mp3s. The total filesize of all this music is a little over 30mb.

Unfortunately, this seems to lead to two problems:

1. Unity takes forever trying to reimport these audio files whenever we restart it.

2. The builds we produce are, well, 30mb bigger than they otherwise would be.

Maybe these are unavoidable problems, but I want to know if anyone has any advice, especially as we eventually plan to have different mixes of songs to switch between based on character position --- music seems like it's going to take up an increasingly large fraction, possibly even a majority, of the game.

Thanks!