SUMMARY
The "plasmashell: Playback Stream" seems to be using up direct audio output device for audio output, causing audio players unable to use the same direct audio output to bypass pulseaudio server for bit-perfect audio playback. Killing plasmashell manually solves this problem with audio playback.


STEPS TO REPRODUCE
1. Install and run Deadbeef through command line.
2. Set "Sound/Output plugin" to "ALSA output plugin", set "Output device" to something like "HDA Intel PCH, ALC 1220 Analog Direct hardware device without any conversions" (Or something alike, according to the correct audio hardware installed on your computer.).
3. Attempt to play a song.

OBSERVED RESULT
Command line output error message: 
"could not open audio device (Device or resource busy)", and no sound is played.
Possible additional error messages: 
"ALSA lib pcm_dmix.c:1108:(snd_pcm_dmix_open) unable to open slave"
After manually killing plasmashell (with "killall plasmashell"), music playback using the same output setting proceeds normally as expected.


EXPECTED RESULT
Audio playback proceeds without problem.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Kubuntu 19.04
(available in About System)
KDE Plasma Version: 5.15.4
KDE Frameworks Version: 5.56.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION
Under Audio Volume setting in KDE (Settings/Multimedia/Audio Volume), under "Applications" tab, two applications can be seen in playback group, which disappears after "killall plasmashell", they are both named "plasmashell: Playback Stream". They appear even when no audio is played or heard. After manually killing plasmashell with the method above, if restart plasmashell, these two applications in audio setting immediately re-appear.