I'm currently working on a song, Tom Waits' Invitation To The Blues. The tempo is slow, and it varies quite a bit. This is a feature, not a bug.

I recorded audio of myself playing the song with a piano sound, varying the tempo. The next step was to use arps to record drum and bass tracks that kept time with the varying tempo. After a full day of hitting my head against The Wall That Is Cubase (8.5 Pro), I figured out how to do this. It was difficult enough to figure out (at least for me) that I thought it might be useful for others to hear about.

1. Do a Tempo Detection on the audio. This will add a Tempo Track to your project.
2. If necessary, adjust the detected tempo using AudioWarp, which is a fancy graphical way to make changes to the Tempo Track.
3. Do a Set Definition From Tempo. This sets up the audio so that Cubase can change its tempo.
4. If you play the audio after all of this, the tempo will still vary.
5. Turn the Tempo Track off. Now when you play the audio it will play back at a constant tempo as specified by the Cubase project.
6. Use arps to record bass, drums, etc at that same constant tempo.
7. Do steps 1 and 3 to this new audio.
8. Turn the Tempo track back on. Now when you play the song, the tempo of the tracks you just recorded will vary along with the tempo of the original audio you started with.

   You can download the Cubase project I used for experimenting with this here.

   I've noticed that audio events sometimes jump around in a seemingly-random way (i.e. their start point moves, their length changes) when I turn Musical Mode on and off. I haven't yet been able to figure out what the deal is with that. If anyone knows, I'd be happy to hear about it.

   There seems to be a Hitpoint-related bug in Cubase 8. In the piano audio I recorded, the last note didn't automatically get a Hitpoint. I was able to fix the problem by adding a Hitpoint manually.