Ah, I see.

/usr/share/policy/etc/rx51/syspart.conf file is what needs to be handled. It's a simple conf file, check group [classify system] and add app path to it. Someplayer (and QRadio) already adds itself there to get CPU cycles.

Doing that you get sound even when pressing power button.

EDIT: there is also multimedia group, but with system group audio stutters are limited to the minumum.