Mixing digital and analog audio inputs to digital out
We have a configuration at work where we use a laptop connected to a TV via HDMI, which is then connected to a soundbar via SPDIF.
We have some con calls where we need to mix sound from the phone system (presented in mono RCA) and sound from the computer (for video clips) to the soundbar.
The reason for this is so that the presenter participants can hear responses from callers on the phone and sound from video clips play over the soundbar (so either the video or a caller would be heard). I'm sure we could probably somehow plug the phone system into the PC and mix internally, but people typically just use their laptop, so trying to deal with several OSes, OS versions, and hardware variations would be a nightmare. I n order to simplify things, we would prefer to mix externally.
Worst case, we could revert to simple analog audio from the laptop, but that would be another cable to plug-in and a different config for user to remember (as they normally just use HDMI)