Some features are disabled in mute-mode to make sure you will not accidently save, download or comment on a different jamtrack from the one you are currently hearing. You may either lift the mute-mode to download or interact with the currently displayed remix, or move to the currently playing track with the green GOTO button.
Cross-platform, but single chord. I recorded an Ample P-Bass (virtual instrument) on an evaluation version of Reaper on Windows, then went back to my Ardour (on Linux) to add a Hi-Hat, a Salamander piano, and a Yoshimi DX Rhodes.
Please feel free to cut & slice if you find this useful. HD track should be bass only, with count in.