Jolla is being deaf and dumb to bugreports, even regressions 
But it seems that I have found a temporary solution. 
- Sennheiser PCX550 is an advanced BT headset that supports multiple A2DP codecs. The working simpler JBL headset only supports the basic SBC codec.
- In order to use AptX, LDAC, AAC etc. SailfishOS needs to have bluez5 and pulseaudio-15.x Source
- pkcon search pulseaudio tells that even the current SFOS 4.3 provides only the outdated pulseaudio (pulseaudio-14.2+git4-1.7.1.jolla.armv7hl (installed)) ! So, no hifi sound for sailors
- I supposed that when bluez5 establishes connection, the PXC550 expects AptX, but gets smth else or nothing… So, I tried to force SBC codec and it worked.
Go to
/etc/pulse/default.pa
and add the following line to the end
load-module module-bluetooth-discover a2dp_config=“sbc_cmode=dual sbc_min_bp=53 sbc_max_bp=53 sbc_freq=44k”
Restart pulseaudio as user (pulseaudio -k) or simply reboot the phone. reconnect the nicer headset and enjoy 1/2 of it’s sonic capabilities.