Combining two video clips with synced audio.

Others may have alternate ways to do this - this is how I might tackle your situation.

Can't think of any fast and easy ways to match the tempos other than trial and error.

If you want to actually use the audio for the 'sped' up clip I'm not sure how natural that will sound but when you speed up the clip - though tick the 'maintain audio pitch' check box in the 'Speed/Duration' dialogue window. You could use markers as a guide to tempo and place the faster tempo clip above the slower one and then adjust the speed until the markers line up. You may find that over time you will still lose sync between clips anyway - just the nature of live performances.

If you are only using the slower tempo audio you may find, assuming the tempo is not too different, that you can edit in the 'faster' clip and not 'see' any serious sync issues. It does depend on how short your edits are.

As to mixing in 4x3 footage ... I'd probably scale the 4x3 so it fits the 16x9 frame and then use 'position' controls to move the frame down (or up) to get the best framing. Here's hoping your 4x3 footage has enough resolution to allow this.

Have you bitten off more than you can chew ... possibly 🙂

This is a potentially problematic edit on a number of fronts.

Hope this helps.

One more thing - choose 'optical flow' when changing the speed of the clip. This will 'smooth' out the frame rate. keep in mind this can introduce some picture artifacts ... but that's anther story. Optical flow will require you to render your sequence to see the end result.