It’s not trivial, but after you get it, it’s not that hard.
You only use one track for the tempo detection. That’s the track you’ll extract tempo info from.
When you perform this extraction, it will set the project tempo track accordingly.
Ah, right… I’ve forgotten one very important part. Select the drum part and use Audio -> Advanced -> Set definition from tempo. Select “project only”. This will imbue the found tempo into the track.
When you then change the tempo, this track will warp accordingly.
If the guitar part was synced with the drums, you can use the same “Set definition from tempo” on it, so it warps together with the tempo.
I’ve once had to made a proof-of-concept track for a medley of songs, each in a different tempo, and “Set definition from tempo” saved me here.
So here’s the deal:
If drums and guitar were synced (what I understand is the case):
- Extract tempo from drums.
- Set definition from tempo on the drums and guitars.
- Set the tempo to whatever you want.
If drums and guitar weren’t synched:
- Use tempo detection on the guitar part / audio warp according to fixed click, might be messy and a lot of manual work.
- Set definition from this tempo track.
- Just play your drums.