Anyone have this problem?

Not sure I can help but if you are hooked up through HDMI, there are protocols that look for the next device in the chain. Your devices or processor may have a setting that turns off the change of devices. 

I have this happen occasionally (not Mac, I have Meridian) it seems that the audio handshake where it chooses the protocol fails. Where it is deciding DTS, or DD, or...

russ69....I am using HDMI. I have searched through settings for an option that might help the situation and cannot find anything.
 
ghdprentice...I agree with your possible diagnosis but cannot find a way to stop the behavior. My ten year old processor I was using did not have this issue.

Have implemented the latest firmware upgrade?You should not have this issue IMO
It is like the processor , when changing channels, stop using the former audio format . Then , it is not able to set the new format ( that 
may be the same ) . You have to go back and forth for the processor
to set the audio format.
Ask you dealer or communicate directly with McIntosh.
Go to McIntosh MX123 forum 
https://www.avsforum.com/threads/the-official-mcintosh-mx123-a-v-processor-thread.3099788/

maxwave....Yes, it has the latest firmware. I agree I should not have the issue, but this is the second MX 123 with the issue. I returned the first unit and got another. I assumed the first unit was defective but now I'm thinking it may be a design flaw. I have spoken with both the dealer and with McIntosh directly and both say they have not heard of this issue. I want to keep it because I am pleased with every other aspect of the processor. I will look to join the avs forum and post the concern there. 
   Regards,
         Steven

It great when no one’s heard of (seen) something that is inplain sight.

Have you tried switching hdmi cables?  I know it sounds dumb and “digital is digital” but just today was trying everything to get a Nintendo switch console working and finally went from the cheap hdmi cable (which I know does work) to an older but very expensive at the time Monster cable and boom, problem solved. 

esthlos13....I switched HDMI cables, I switched the cable boxes, I tried a different HDMI input, but the problem persisted.

I am having the exact same problem guys, has anyone figured this out? For me it works better when I’m using Apple TV as opposed to my Sony television.

Hey OP,
I'm wondering if you are using a Sony TV?  I'm not familiar with your Mac, but remember reading about people having issues between Sony TVs and Oppo players.  It might be a setting with your TV.

Do you remember what the setting was?

Hello
I purchased my Mcintosh Mx 123 two months ago and experience the same problem. I also have to go back and forth to get the sound back. Today i have a new problem in that i dont have any sound at all. I have tried everything, unplugged the power, checked firmware update all to no evail. Will have to contact the dealer and see if they can come out and help. Anyone els with this type of problem?  
Regards
Jannie