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I've been using crouton through my Acer 720c Chromebook. In addition, I have an external monitor that I like to use with it, ASUS VS247. This particular monitor has an audio pass-through but NO speakers. This confuses Chrome OS into outputing the audio through the monitor.

This is normally not at all a problem, since I can switch it in the settings. However, unless the monitor is at native resolution on the Linux chroot, the audio will flip back to using the monitor as the sound device.

I've tried messing around with alsamixer in both the Chrome OS shell and the Linux chroot to no avail. Is there any way (preferably in Chrome OS, but not necessary) to always make the Chromebook speakers the DEFAULT audio device?

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Interesting, I haven even thought about looking if alsamixer is available in Chome OS, udev tools are also available. You should try to create a udev rule to ignore this device as described in Tell PulseAudio to ignore a USB device using udev by Jamie Nguyen (or blacklist the kernel module, suggested further below):

Create a udev rule

Your system should have udev rules already defined in the /lib/udev/rules.d directory, such as 90-pulseaudio.rules. There should also be a /etc/udev/rules.d directory where you can define your own rules.

The rules are parsed in lexical order. Create a file called /etc/udev/rules.d/89-pulseaudio-usb.rules, so that it’s parsed just before 90-pulseaudio.rules (though you may have to experiment with the numbering if things don’t work as expected).

Open the file in your favourite editor and create a rule. Use the idVendor and idProduct numbers from earlier to uniquely match your USB device, and then set an environment variable that tells PulseAudio to ignore the device:

ATTRS{idVendor}=="1852", ATTRS{idProduct}=="5110", ENV{PULSE_IGNORE}="1"

[…]

Reboot your system and PulseAudio will no longer acknowledge that your USB device exists.

… or How to reload udev rules without reboot? The problem of course is that you are not asking about a USB device, it's likely to be HDMI. The following rule might work but I have not HDMI audio capable device to test:

SUBSYSTEM=="drm", ACTION=="change", ENV{PULSE_IGNORE}="1"

You can also try to blacklist the corresponding kernel module as suggested in this example. I think you cannot make these changes to Chrome OS by default, I have found no stateful partition in the output of mount that would let me make such changes so you have to do it in chroot.

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