Cambridge Audio Azur 340R - Cambridge Audio Azur 340R

I would repair the volume control motor driver circuit.
They often fail.

https://elektrotanya.com/cambridge_azur-540a_v2.0_sm.pdf/download.html

try:
http://www.excelia-hifi.cz/cambridge-audio/manual/cambridge-audio-azur-550a-650a.pdf
Is the user manual try the OEM for the service manual, most OEMs will require you to be qualified to do the repair.

What ohm rating does the receiver have and what ohm speakers are you connecting. If the receiver is rated for 8 ohm speakers and you are connecting 4 ohm speakers, you are overdriving the amplifier. This will cause the protection to trip. Also make sure your speakers polarity are matched. If they are out of phase this may also cause a problem.

http://www.acousticpsychos.com/Files/Cambridge/Cambridge-Azur-340A-int-sm.pdf

Probably an issue with the servo control / laser . Needs a tech to fix.

In any 'connection' question there are at least two participants and specific answers require specific details about all components in the path.

Does the center speaker work for just speaker testing? Your unidentified BluRa player has to know there's a Center Speaker, too.

Digital bitstreams don't arbitrarily drop out one channel.

Cambridge Audio will be the only people that have spares
Check their website

have the volume control cleaned by a spray of electronic contact cleaner. If the carbon coating material is already wiped-out due to use, have the part replaced

best connection from pre-out to aux in.