Add support / docs for playing audio file

[grokify]

Support playing audio file in the following scenarios:

1. before connecting the user and
2. after connecting the user.

SIP.js supports this so it would be useful to expose this and provide docs for how to do this.

More info is available here: onsip/SIP.js#363

[egreenmachine]

Are you trying to play a file on the web page or stream it over the media stream?

Could you link the SIP.js support that you were looking at?

[grokify]

I'm try to stream an audio file that is accessible from the web page where RingCentral web phone is being used.

The SIP.js support I'm looking at is linked above: onsip/SIP.js#363

In that issue, it is mentioned that SIP.js will support streaming a file when using WebAudio instead of getUserMedia for the MediaStream. I would like to use this same functionality via RingCentral.