i have volumio installed on raspberry pi 4,
how can i Sets HDMI and AUX at the same time?
Look up the alsa multi plugin 
EDIT: This isn’t a normal use case for Volumio, so it’s not supported out of the box. Be aware that the two sources will not remain synced forever!
how do i install and use alsa multi plugin?
Please post back when you figure it out, others might might find it useful
I found it but unfortunately it does not work
Multiple simultaneous outputs for a multi-zone receiver - Diy and Tweaks / Diy and Tweaks - Volumio
Oh that is a great find! With some tuning/adjusting to map to your cards, it should work - define “unfortunately it does not work” ?
How do I find the correct adjusting of the card?
And I do not have an external sound card or anything like that
Out of the Raspberry Pi comes HDMI and an AUX cable from the headphone port
I made a copy, pasted to the text written there
And I only hear HDMI
In the file: sudo nano /usr/share/alsa/alsa.conf
Defined by me:
defaults.ctl.card 0
defaults.pcm.card 0
Is there anything that needs to be configured in the volumio UI?
I won’t work easily as Volumio adreeses directly to one hw. Tweaking is not esay… But wait, a new arch for alsa is comming in volumio and it it will be easy to to write a plugin able to do that…
to find available output :
aplay -lOK, now what?
**** List of PLAYBACK Hardware Devices ****
card 0: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
Subdevices: 4/4
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
card 0: ALSA [bcm2835 ALSA], device 1: bcm2835 IEC958/HDMI [bcm2835 IEC958/HDMI]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 0: ALSA [bcm2835 ALSA], device 2: bcm2835 IEC958/HDMI1 [bcm2835 IEC958/HDMI1]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: Headphones [bcm2835 Headphones], device 0: bcm2835 Headphones [bcm2835 Headphones]
Subdevices: 3/3
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
I don’t mean to sound rude - but don’t expect to find a copy paste solution for something that is not considered normal operation for Volumio.
If I were you, I would do some reading up on the ALSA system and how to define playback devices…
Honnestly, as I said, it is complicated… Feel free to have a look at : https://github.com/balbuze/volumio-plugins/tree/master/plugins/audio_interface/volstereo2mono to see how redirect output in volumio to use asound…
Yes. Volumio 3 uses the new Alsa arch. So, a plugin should be able to redirect the output to an other hw
See Custom ALSA Contributions - Volumio Documentation