Hi
As multiple subject have a touch of sound / video streaming ability in GoTo training, hereby some ideas if you need sound to a video or sound from another application, media player etc. you can try
A)
A virtual web cam software including sound by a sound mixer software.
Or

Share a screen with the video running (if this runs smoothly as GOTO might have limitations on shared screen with frames per second FPS showing video) and again use a soundmixer software.
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For A)
Video element, using a virtual web cam as input in GoTo training web cam.
Streaming by a virtual web cam, "OBS" software is for free, which can combine web cam, screens etc. as well you can set up green screen and make it "live". By adding a plug in in OBS for output by virtual cam (I have tested this on Win 10)
Alternative video element virtual web cam as input in GoTo training.
Stream by Manycam software which also includes a virtual web cam output combining various inputs etc...
(but has license, this can chosen as a one off or periodic payments).
This software seem to be a bit better for the output of virtual web cam quality compared to OBS. (I'm using this now on Win 10, with green screen, 16:9 screen) It's a bit tricky to start/connect this virtual web cam in GoTo, you need to create two virtual web cams in manycam and switch between the two when initialising the input choice in GoTo, but then otherwise running fine, and you can set ouput quality resolution, FPS and bitrate fitting with reasonable streaming level to GoTo like 720p and FPS like 20 - 25 -30 etc...)
For A) and 
A soundmixer application, using the application software output as sound/mic input to GoTo training.
VB software "Voice Meter" is a free software (donations) you download this and as well the VB Audio Virtual sound cable is required, then you can mix you mic and sound from your PC. By the virtual sound cable you pick up sound as you normal hear in your speakers from application running on your PC, and can then be mixed with other hardware inputs (like microphones), which level/mute you can control each for what you like in the output from VB Voice Meter, you then select the voicemeter as input mic in GoTo...
I.e. you can as well stream music by Media player or VLC etc.. as mic input from voicemeter in GoTo ...
The quality of sound off course depends on bit rate in GoTo, but it seems for ordinary listening it will do...
(I have done this in Win 10)
Note:
There's a lot of software out there which can do this, likely some better or worse than the mentioned, I'm not writing to promote the above mentioned softwares, nor rate which ones is best, and have no idea if it runs on MAC. It's just some examples I have tried out. Hope it helps....