What factors determine synchronous signal transmission ability of a channel?
Not sure what exactly you mean by "synchronous". Most digital transport mechanism (such as Ethernet or 802.11) are "asynchronous", i.e. a continuous data streams  are chopped up in packets, the packets are transmitted whenever the connection allows it, the packets are assembled in the correct order at the receiving end and then turned into a continuous data stream again. Since it's not known when exactly the packet can be transmitted, there has to some buffering at the receiving end so there is some data available even if the next packet comes in late. This results in a certain amount of "latency". In the case of Bluetooth + VoIP you are probably looking at several 100 milliseconds to a few seconds of overall latency.

You may also be concerned about inter-channel sync, i.e. do both channels of the stereo stream stay time aligned. This can easily be achieved by interlacing the channels to create a single stream.

This all depends heavily on the specific implementation: Bluetooth supports many profiles. For your application you probably want the A2DP profile. This uses the SBC codec and there are dozens of ways to configure the codec ranging anywhere from high quality to totally unusable. Both parts of the interface need to agree on the codec implementation.

There are also many different VoiP protocols and codecs out there. Most operate in the 10kb/s-40kb/s range which seems way too low for you application, so you may need to create a custom protocol.