I've seen this before, when logging in with the same account between two systems that have different sound hardware.
Try terminating kmix, delete the two kmix* rc files, then start kmix.
Good thought. I killed it, removed the .trinity/share/config/kmixrc and restarted. No change and no regeneration of kmixrc. I'll keep poking at the underlying sound system to see if there is something there causing the issue.
After deleting the two kmixrc* files, the default should be to autostart. If kmix is not autostarting, then try manually starting from the command line.
Darrell