Note that the pro version of QuasarMX (enabled with a login in the free app) adds an equalizer, mixer, enhancer and playlists (no pitch setting though, but many sound adjustments possible). I just realized the UI supports keyboard navigatioin (arrows, Ctrl+L to select the search field, etc.).

@Mosen, Fellfrosch, actually I cannot play Twitter videos in Piepmatz and Sailfish Browser, but realized I can play Youtube videos in Sailfish browser. Not in LLSVideoplayer, Microtube. or LLSVideoplayer, Media Player and Pico Player will also fail on local files. Kodi plays local files.

It's more complicated than I thought.