that works well and I have done it years ago but what is needed is the audio interface unit to tranlate the digital or analog signals to respective audible voice output.
I had used a telephone 900 mgrz TX in the sub that lisned on an open micke to variouse voice modules that when triggered played back there pree recorded info.
It worket very well but did take up some space.
Now with smaller voice recording modules one can make a sub talk back to you all of its functions and added to the video audio signal might work but the signal loss of the video might also lose the audio.
I would go back to the original just FM or AM radio signal for great reception even at depth.
Micro size TX units are available as Bugs,and I would call it : Bug your Sub Telemetry system and wouldn't cost very much.