Hello meeksy -
Go to the web interface of one of the codecs. Setup / Configuration / Audio.
Then scroll down to the HDMI input that you are using for the computer. Make sure that Mode is set to On. There is also a setting "VideoAssociation MuteOnInactiveVideo"; you could try turning that to Off, although it is not supposed to matter if you are choosing that input as a content source.
Also, you could try downgrading one of the SX80 or MX300s to CE9.12.4 as a test.
If CE9.12.4 works, but CE9.13 does not, then you would want to open a TAC case.