Eugh. Nice question.
Simple answer....you don't.
Complicated answer....elastic audio.
Enable for all tracks, consolidate all your regions then place a marker at the end of the session, on the grid.
If you now change the session tempo, the midi regions ( and markers ) will move/stretch.
That'll tell you how far to stretch the consolidated audio tracks.
Go in to warp view and stretch your audio tracks so that they are in line with the marker again.
Can't guarantee it'll sound great, but it'll work.