Okay, I got into the Terminal window and tried using sudo in order to install the PPA for the DeaDBeef audio player. I got this line:
[sudo] password for jaguar:
I assumed it's the same password as for the RTOS, but the terminal window wouldn't let me enter it; nothing from the keyboard registered on screen. Any ideas on how to proceed?
You're doing everything right, so I don't know. I'm pretty sure DeaDBeef is not compliant with JACK, so you won't be able to adjust the latency settings with the JACK interface. It is the one Linux player I've found that usually works without a week of troubleshooting after the install, though you'll probably take a hit on sound quality.