Both the regular Bela starter kit and the Bela Mini have stereo audio in and out, i.e 2 input and output channels. We bundle audio adapter cables with the kits which allow you to connect using stereo 3.5mm audio jacks.
The sample rate for these inputs and outputs is fixed at 44.1KHz for both.
With the audio expander capelet you can convert the regular Bela starter kit's analog i/o to audio, giving you an additional 8 audio channels at 22.05KHz, or 4 channels at 44.1KHz, or 2 channels at 88.2KHz.
With the CTAG capes you get a lot more audio channels, 4 in 8 out with the FACE or 8 in 16 out with the BEAST.
Whether you need external power depends on what kind of speaker and microphone you are using. The regular Bela cape has speaker amplifiers on the board which work when you supply 5V through the barrel jack of the BeagleBone. Most speakers will require some form of external power, such as a battery.
Phantom power is only required for certain kinds of microphones, so look at the data sheet for the one you are using.
There is no support for HDMI, instead you connect your Bela to your laptop or desktop computer over USB and use our browser-based tools for debugging and development. Ethernet is also supported on the regular Bela kit.
You can power Bela with a battery connected to the USB port or barrel jack when it is not connected to a computer. Cables for connecting to the computer are included in the kits, they are regular USB cables (mini for the regular Bela and micro for the Bela Mini).
You can use whatever c / c++ libraries you like.