Audiobitrate parameter is actually specific to video and our (to be released) audio recorder embeddings only. The player would never check nor use the value of this field, rather play it as it is set in video itself.
Now I always suggest leaving it to auto. The reason is that our system does checks (video and audio) to see how things should or are best to happen and then might decide that some specific audio bitrate has to be used for that one recording while a different in another for optimal performance or to even be able to record.
We however decided to make it available just so you can use any value that you might prefer instead.
My suggestion (if you would go down the road of setting it up) would be to try and use it to see how low or high you want it to be.
I do want to mention that audio data is actually a very small aspect of recording. The video data is much larger in size. So while you will achieve some difference in size, unless you optimize your video the audio optimizations would not lead to huge improvements.
Now I always like to mention one more detail when it comes to streaming. Streaming is only good if you do not really care about the quality of the video recorded. The streaming protocol (it's nature - and hence how it is implemented in browsers) makes it possible to send massive amount of data quickly, however it has to be aware of the currently available network speed.
So if you have someone on your network start playing some network game, watching Netflix or anything like that at some point during recording you are likely to notice drop in quality at least for a moment (influenced directly by your local home or office network gear and setup).
Protocol will be OK with HD resolutions however the video and audio bitrate is then managed automatically by browser (not our system). By this I mean that your setting would be seen as maximum value and protocol would choose anything between 0 and that number during recording.
If you are not using streaming, rather regular recording, then you can set the audio and video bitrate properly and this would be seen as maximum and minimal requirement and would drop only if absolutely needed (like camera unplugged for example).
If you want the time of upload to go faster, and keep the quality and optionally play with bitrate, my suggestion would be to also use "localplayback" as true. This will make the video show up as preview during the upload stages.
Also a reminder that once "verified" event fires, you can remove the embedding, switch the page and any other action you prefer to do as you no longer need to be on the same page, or have the recorder available.
Hope this helps :)