https://wiki.libreelec.tv/
https://libreelec.tv/
https://kodi.tv/

As CC mode runs without an RME driver and CC mode is generally known to work for Windows and macOS,
this sounds to me like an issue of this special Linux Distribution.

As there is no "standard" Linux distribution it makes things a little bit more difficult.
I would cross test with other Linux Distributions or ask the developers of Kodi and libreelec.

Or maybe somebody other in this forum has more Linux experience in terms of 5.1.

For cross-testing with another Linux I would use a Distribution that is known to have a good audio support.
Maybe this KI output gives you some ideas
If it works with another distribution it may help you to find the root cause for libreelec.

KI output

Modern Linux distributions handle multi-channel audio (such as 5.1 surround sound) and USB Class-Compliant (CC) recording interfaces through PipeWire. PipeWire replaces legacy PulseAudio and JACK frameworks, automatically configuring multi-channel output streams while allowing pro-audio applications (DAWs) to capture multi-input USB class-compliant recording interfaces at low latency.

Key Requirements & How Linux Handles Them

    5.1 Surround Sound Output:
    PipeWire exposes multi-channel profiles (e.g., Analog Surround 5.1 or HDMI 5.1 Surround) directly through standard desktop volume controls. Up-mixing stereo content (music/YouTube) to 5.1 is done via PipeWire configuration without needing manual JACK routing.

    USB Audio Class Compliant (CC) Interfaces:
    Modern Linux kernels utilize the built-in snd-usb-audio driver. Class-compliant hardware (like Focusrite Scarlett, MOTU M-series, Audient iD, Behringer UMC, and RME in CC mode) works out of the box without requiring vendor-specific drivers.

    Low-Latency Audio Recording:
    PipeWire includes a pipewire-jack drop-in replacement, allowing software like Ardour, Reaper, Bitwig, or Audacity to interface directly with multi-channel USB devices at low buffer sizes (e.g., 128 or 256 samples).

OS recommendations

Distribution: Ubuntu Studio
Audio Server: PipeWire + Low-Latency Kernel
Realtime Tuning: Pre-tuned real-time permissions
Best For: Production-ready DAW workflow out of the box.

Distribution: Fedora Workstation
Audio Server: PipeWire (Default)
Realtime Tuning: Out-of-the-box system limits
Best For: Plug-and-play general desktop with great audio support

Distribution: Manjaro / EndeavourOS
Audio Server: PipeWire
Realtime Tuning: Easily configurable via Arch AUR
Best For: Users wanting bleeding-edge PipeWire and Linux kernel updates

Distribution: Pop!_OS
Audio Server: PipeWire
Realtime Tuning: Great hardware compatibility
Best For: Mixed use: gaming, media consumption (5.1), and music recording

Details

1. Ubuntu Studio

    Why it fits: Purpose-built for multimedia production. It ships with a low-latency kernel, pre-configured real-time audio permissions (realtime group limits set), and PipeWire pre-configured for low-latency routing out of the box.

    5.1 & CC Handling: Includes control panels like pavucontrol and QpwGraph to manage surround-sound profiles and visual audio routing for class-compliant multi-channel interfaces.

2. Fedora Workstation

    Why it fits: Fedora was the first mainstream Linux distribution to make PipeWire the default audio engine. It provides upstream updates to PipeWire and Linux kernel versions, bringing hardware compatibility fixes for class-compliant USB chipsets quickly.

    5.1 & CC Handling: GNOME Control Center automatically detects 5.1 speaker configurations over analog jacks or HDMI/DisplayPort, while PipeWire handles Class-Compliant USB recording interfaces seamlessly.

3. Manjaro Linux (KDE Edition) or Arch-based

    Why it fits: Rolling releases ensure access to the latest Linux Kernel (e.g., Kernel 6.x+) and PipeWire packages, which can be critical for hardware compatibility with new USB 2.0/3.0 audio interfaces and HDMI eARC surround receivers.

    5.1 & CC Handling: Plasma Desktop (KDE) has one of the best GUI speaker layout panels for assigning 5.1 channel maps, testing speaker placement, and managing multi-channel interface inputs.

MINIX PC Intel n350 running LibreELEC 12.2.1 (Generic.x86_64)

RME Fireface UFX II operating in Class Compliant USB Mode

The Problem:

Playing 2.0 audio works perfectly. Playing any 5.1 video causes Kodi to hang/crash after about 10 seconds and not play any sound. After such a hang, also gui sounds don't play anymore until reboot, which they did before the hang.

What I've Tested:

The RME in CC mode presents itself strictly as a 24-channel device (hw:1,0).

speaker-test -D plughw:1,0 -c 6 -t pink via SSH works flawlessly, proving the ALSA driver and plughw can handle the 6-channel stream and pad the remaining 18 channels without crashing.

Inside Kodi, if I select the ALSA driver, 2-channel works, but 5.1 crashes ActiveAE.

We tried forcing asound.conf profiles (using type route to map 6 channels to the 24-channel S32_LE slave format, and type plug), and forcing Kodi to use these via guisettings.xml. It resulted in either the same 10-second crash or an infinite loading spinner.

Disabling passthrough and locking Kodi to Fixed / 48kHz did not stop the crash.

It seems Kodi's ActiveAE is deadlocking when trying to pass multi-channel PCM to this 24-channel Class Compliant interface, even though native ALSA handles it fine. How can I bridge this gap?

Anyone tried to connect Libreelec with RME interfaces?

Thank you for any advice.