[Vantskruv]

Anyone aware of any alsa problems?

Hence, I did install this addon, launching retroarch, or ROMS directly with retroarch, and I did not get any sound.
I think Kodi is locking the alsa-device and therefore blocking sound from retroarch.

Any solution for this? Using pulseaudio is not a option as I've read it does not support all audio-formats with passthrough.

 Yes! Kodi lock the sound if you use determined option on sound settings. I don't remember what setting is (on work right now), but it's quite simple to resolve: Set an option on sound settings and launch a rom, if the sound is muted, exit retroarch, go back to settings e select other setting. Repeat until get sound on retroarch.

Cheers    

I'm not aware of such option in Retroarch, but I recomend you (Vantskruv) try it first.

ALSA by default allows mixing (several apps accessing the same sound card) for analog devices, but not for digital ones. For example, if you are using HDMI (a digital device) to output your sound, then you have to create a dmix plug in your .alsarc configuration file. Please google a bit, there are many tutorials out there (in particular, check the Archlinux wiki is always useful). ALSA configuration may be painful but in the end you can get any result you want. For example, if you have a 5.1 audio system, you can create a virtual sound card for your emus and tell ALSA to output left and right channels to the left/right speakers, respectively, then mix both right and left for the central speaker, and apply a low pass filter for the subwoofer, all at the same time you are watching a 5.1 movie in Kodi with audio passthrough. OR, you may want the virtual sound card to output left/right sound only in the left/right front speakers, to create a more retro experience.  

Thank you very much both of you.
The settings in Kodi did not help, but I guess than as @Wintermute0110 said it would not help if you send sound through HDMI, which I am doing (pass-through).
Also I did not know that you could mix up different sources in ALSA, I will investigate more about my ALSA settings when I get more time.

For now on OpenAL did work, but only if game is started from Retroarch.

Another thing worth to note, not related to sound, is if using the same gamecontroller for playing games in Retroarch, as navigating in menus in Kodi, Kodi will still read input from the gamecontroller while playing the games. Ironically, and unintentionally, I managed to disable peripheral joysticks in Kodi while playing a game in Retroarch. When exiting Retroarch, I noticed Kodi was in the input menus for whatever reason, and I could not navigate at all.