[storn42]

For the time signature changes, the best thing to do is to change the bpm to keep 4ths on the down beat (im sure you know this). The change in bpm depends on when you move the bpm from. You can either use the waveform to line up the beats or use big brain math, but it usually tends to be some sort of easy 2x, 3x bpm. Bloody mary kate and ashley for example was colored by hand. It was all simple enough to map out. (2x bpm when ever a beat was skipped) its not always easy numbers, but there should always be a logical mathmatical relationship between the 2 bpms when beats are dropped.

For songs with multiple bpms i find AV 's auto bpm to be useful. It can usually find multiple bpms in a song so while it may try to give you 1 constant bpm for the whole song, it tends to atleast be in the ballpark for other bpms to try when you encounter a bpm change. Though honestly, as someone who mapps in AV, i find it quite easy to manually figure out bpms. Listening to the bpm ticks can also make it easier to sync. The last is advise i can give you is to look up/find the bpms for the songs in the mashup individually. Syncing might be annoying, but it should be easier to sync If you know thr bpm going into it.

For songs with variable bpms(not bpm changes), i dont think there is an easy way.the only thing you can really do is try and find the most consistant bpm of the song then manually move the beats back in line. AV once again usually gives a good idea of the average bpm of the song, but nothing can stop you from having to manually fix beats.

Honestly, bpm stopped being an issue for me when i started using AV. It just made life a lot easier. Hope this helped.