ffmpeg seems not to be able to do what I need and aplay -L wasn't capable of finding Bluetooth sources. But with pactl list sources short I finally found the A2DP-source.
Anyways, after messing around with sox, I finally got an audio signal from default-device to bash, but it was stuttering as I used the following command.
$ sox -d -t raw -r 22.05k -b 8 - gain -5 | fm_transmitter -f 87.6 - After I thought of the sampling rate at first place being 44.8k, and the piped rate being 22.05k - which is not perfectly dividable - I tried different tempo rates.
I found tempo 0.97 being fine for me, still it stutters in the first seconds but then it works.
The only issue is, the tempo reduce adds a slight stacking delay - which isn't perfect nor real-time at all.
$ sox -d -t raw -b 8 -r 22050 - gain -5 tempo 0.97 | fm_transmitter -f 87.6 - Playing: stdin, 22050 Hz, 16 bits, mono Input File : 'default' (alsa) Channels : 2 Sample Rate : 48000 Precision : 16-bit Sample Encoding: 16-bit Signed Integer PCM In:0.00% 00:07:15.97 [00:00:00.00] Out:9.90M [ =====|===== ] Hd:4.4 Clip:0