Evening all, and thanks for reading my question.

Scenario:
Filming at a sports event with the stream going to a remote location (using VMIX) where graphics and stuff is added to the SRT feed (using VMIX as well), and then sent to YouTube.

Main audio is carried on SRT, which then will be sent to output and to YouTube, but we would also like to add an additional audio feed for the onsite producer to talk to the remote site, but that audio would not be broadcast.

I understand SRT can carry 16 channels, which VMIX covers in 8 (I presume stereo making it 16). But I cant work out how to

1) Add the additional producer audio feed into the SRT Out (although we may be able to use Blackmagic SDI Audio embedder)
2) Separate them out at remote - so Main audio goes to output, and i can listen to the the onsite producer)

Ideally would also like to feed back my audio, but SRT not an option (I believe) as would need to alter router settings onsite (can you even have SRT streams going in opposite direction). Possible to use NDI

configure your sending SRT audio channels in the vmix output setting page. Click the gear on the appropriate 1-4 output to get to the page where you see the SRT URL setting. Top of that page is a dropdown for audio channels, default is Master which will only send 2ch from the Master Bus. Change this to MABC to send 8ch from M-A-B-C busses. land your prod mic somewhere other than M bus.

when received at the other vMix you should see 8ch of inbound audio, and can route where it goes in the audio settings of the SRT input tile.

Assuming you are initiating caller mode for this feed since it's usually easier to port forward thru the router on the fixed studio end. You can also use caller mode to get a return feed going the other way in the same fashion. Caller/Listener only determines connection, not which way the feed sends.