Audio source not appearing in pulseaudio but playing over headphone jack

I use a headset with internal USB sound card running on Ubuntu 16.04 with KDE 5.5 and Firefox 56.0 (64-bit). Recently playing music using the Amazon Music web player stopped working.

• Using the same setup, music from other sources, like Youtube, works without problem.
• When I start a track in the Amazon Player, it starts and the progress bar is moving. However, no music is playing over the headphones.
• When I start pavucontrol, no new entry in the playback tab is created for the audio source and there is no movement in the output devices tab.

Now for the strange part:

• When I plug my earbuds headphones into the audio jack, I hear the music Amazon is playing.
• There is still no movement in the devices tab and no entry in the playback tab.
• When I mute the internal sound card (the one that is used for the earbuds), the sound stops.

I have tried

alsactl restore

and removing the pavu config files, but both did not solve this problem. Additionally I installed virtually all pulseaudio component and added the Phonon VLC backend. I tried disabling all devices but my external sound card, but nothing worked.

Does anyone have an idea, what is going on here?

Edit: On chromium, the player works and the source shows up in pavucontrol, as long as I play music from my library (i.e. tracks I 'own' as opposed to tracks I added via Prime 'for free streaming'). The other tracks start, immediately stop and skip to the next track.