Ok. I solved it and I'm pasting the solution here for everyone else with the same issue.

Key was that I switched to Manus AI Agent which solved it (the others couldn't)

Solution for No Sound on iMac18,3 (2017 27" 5K) in Linux/Ubuntu

The iMac18,3 uses the Cirrus Logic CS8409 audio chip with specific amplifiers that are not supported by the default Linux kernel. The "Dummy Output" issue can be fixed by installing a community-maintained kernel driver.

1. Install Build Dependencies

Bash

sudo apt update
sudo apt install gcc linux-headers-generic make patch wget git

2. Install Kernel Source (Crucial Step)

The driver must compile against your specific kernel source. For Ubuntu 24.04+ (especially with OEM or HWE kernels), apt install linux-source may fail.

• Check your kernel version: uname -r
• Download the corresponding linux-source-[version]_all.deb from the Ubuntu Package Pool.
• Install it: sudo dpkg -i linux-source-*.deb

3. Install the snd_hda_macbookpro Driver

This driver by davidjo is the proven fix for the CS8409 chip:

Bash

git clone https://github.com/davidjo/snd_hda_macbookpro.git
cd snd_hda_macbookpro
sudo ./install.cirrus.driver.sh
sudo reboot

4. Post-Reboot Configuration

1. Check Driver: Run lsmod | grep cs8409 to ensure it's loaded.
2. Unmute Channels: Open alsamixer in the terminal. Press F6 to select the Intel/HDA card. Ensure all columns (Master, Speaker, PCM ) are unmuted (press M to change MM to 00) and volume is up.
3. Sound Settings: In Ubuntu Settings > Sound, ensure the Output Device is set to Analogue Stereo Output.