while having an audio stream over USB is possible, it is also hard (see here). The crucial issue is synchronising the two (or three) sampling rates:
- the sampling rate of the codec on the embedded board (e.g.: Bela)
- the sampling rate of the USB transmission (this is provided by the USB gadget driver)
- (optional) the sampling rate of another soundcard connected to the host computer (if needed)
These clocks are all running independently, and while you can set them nominally to the same value (e.g.: 44100Hz), they will never be synchronized (unless you do some magic), and so you will invariably need a sample rate converter at some stage in the chain, which adds latency and lowers quality. Additionally, given how the USB driver that handles this connection is not real-time safe, you'll have to add additional buffering (and latency) in the driver itself to ensure a reliable stream of data.
The above issues are independent of whether you are using Bela or another platform. Other boards with multi-core CPU can have an advantage over Bela in that the USB driver can be handled by a different CPU core than the one that handles audio, and is less likely to choke up when the audio uses a lot of CPU.
If you want to send multichannel audio, this is even harder, as it requires editing the USB gadget driver (which, IIRC, is only stereo). See more details at the thread linked above and the related thread here.