@phg: I was able to reproduce your issue with my BT devices, which connect fine but don’t play the sound. I observed that also the volume button doesn’t change to the volume_music_bt one. Unfortunately, I have no fix for it as I suspect an issue with the 8901.3.A.0045 firmware as the root cause. With the regular Fairphone OS similar issues appear to be reported here. We may have to wait for a firmware update by Fairphone - I’m very sorry 
@developers in the forum and at Fairphone: Do you have any idea what goes wrong and how to overcome it? I have compared my properly working device and the one on the latest firmware. On the working pimped 8901.2.A.0134 firmware logcat briefly says when configuring the BT hardware:
10-12 09:12:30.094 2018 2159 I bt_stack: [INFO:btif_config.cc(798)] write_checksum_file: Disabled for multi-user, since config changed removing checksums.
10-12 09:12:30.654 2018 2439 W bt_btif : bta_av_open_rc: Using the new AVRCP Profile
while the current 8901.3.A.0045 firmware leads to:
10-06 17:50:41.810 2003 2140 I bt_stack: [INFO:btif_config.cc(798)] write_checksum_file: Disabled for multi-user, since config changed removing checksums.
10-06 17:50:41.815 2003 2410 W bt_stack: [WARNING:metrics.cc(773)] LogClassicPairingEvent: failed for 88:c6:XX:XX:XX:XX, handle 65535, hci_cmd 0x000fffff, hci_event 0x0007, cmd_status 000000, reason 0x0fff, event_value 0, error -19
10-06 17:50:41.844 2003 2410 I bt_stack: [INFO:btsnoop.cc(289)] whitelist_l2c_channel: Whitelisting l2cap channel. conn_handle=2 cid=0x0040:0x03c1
10-06 17:50:41.846 2003 2410 E bt_l2cap: l2cu_adjust_out_mps bad packet size: 0 will use MPS: 0
(…)
10-06 17:50:41.863 2003 2410 W bt_stack: [WARNING:metrics.cc(795)] LogSdpAttribute: failed for 88:c6:XX:XX:XX:XX, protocol_uuid 0x110b, attribute_id 000000, error -19
10-06 17:50:41.863 2003 2410 W bt_stack: [WARNING:metrics.cc(795)] LogSdpAttribute: failed for 88:c6:XX:XX:XX:XX, protocol_uuid 0x110d, attribute_id 0x0009, error -19
10-06 17:50:41.863 2003 2410 W bt_stack: [WARNING:metrics.cc(795)] LogSdpAttribute: failed for 88:c6:XX:XX:XX:XX, protocol_uuid 0x0019, attribute_id 0x0004, error -19
(…)
10-06 17:50:41.924 2003 2410 W bt_btif : bta_av_rc_create: Skipping RC creation for the old AVRCP profile