In pavucontrol, I can choose between RV620 HDMI Audio [Radeon HD 3400 Series] and Built-in Audio.
When the former is enabled, videos on (among others) Youtube play way too fast, but this answer solved my problem (though I don't know why). However, when I use Built-in Audio instead of RV620 HDMI Audio [Radeon HD 3400 Series], the mixer in my applet appears to be disabled; the icon is replaced by a blank and changing the volume has no effect, as the applet apparently only relays to RV620 HDMI Audio [Radeon HD 3400 Series]. How do I get an applet to control the volume for Built-in Audio instead?
