RTSP with MP3 fails (GUI issue?)

Windows 64-bit

3.0.16 release

As a test, I am streaming to localhost and using a second instance of vlc to recieve on the same computer

I'm using the GUI, so if there is a configuration problem, its probably a GUI problem. hard to find documentation on the matter as well...

RTP with MP3 works fine. I can hear myself on the microphone.

HTTP with MP3 works fine too.

input:

:dshow-vdev=none :dshow-adev=Microphone (Conexant SmartAudio HD)  :live-caching=300

Transcode profile: MPEG-TS, MP3, 128kbps, 2 channel, 44100hz

output:

:sout=#transcode{vcodec=none,acodec=mp3,ab=128,channels=2,samplerate=44100,scodec=none}:rtp{dst=localhost,port=5555,mux=ts} :no-sout-all :sout-keep

recieve:

rtp://@:5555

log:

-- logger module started --
main: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
main: playlist is empty
-- logger module stopped --
-- logger module started --
main: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
main: playlist is empty
dshow error: no video capture device was detected
dshow error: no video capture device was detected
-- logger module stopped --

RTSP fails:

input is the same transcoding profile is the same (MPEG-TS, MP3)

output:

:sout=#transcode{vcodec=none,acodec=mp3,ab=128,channels=2,samplerate=44100,scodec=none}:rtp{sdp=rtsp://:8888/} :no-sout-all :sout-keep

recieve:

rtsp://@:8888

log:

-- logger module started --
main: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
main: playlist is empty
live555 error: Failed to connect with rtsp://localhost:8888
satip error: Failed to setup RTSP session
-- logger module stopped --
-- logger module started --
main: Running vlc with the default interface. Use 'cvlc' to use vlc without interface.
main: playlist is empty
dshow error: no video capture device was detected
dshow error: no video capture device was detected
stream_out_rtp error: cannot add this stream (unsupported codec: mp3 )
main error: cannot continue streaming due to errors with codec s16l
-- logger module stopped --

mp3 is "unsupported"?

is this a bug or lack of documentation?

from documentation on "mux" value for rtp module:

Only ts is possible for RTP streams. By default, each elementary stream is sent as a separate RTP medium, i.e. no encapsulation is done.

the GUI is not selecting any mux value when I select RTSP with the same transcode profile. is this normal?