If you want to listen to the people sitting in the room with the HD Onyx BT speaker/mic, then you could just connect your cell phone to that speaker by Bluetooth and call your cell phone from another phone (e.g. a landline) and answer the call yourself. (You'd have the cell phone and the land line phone with you.) Then you could listen to the Onyx microphone from the landline. You may want to completely turn down the volume on the Onyx so that it doesn't ring when you dial it.
I don't think there's any way to listen to the Onyx's microphone except via phone call because (unlike a webcam) it was probably only designed to use Bluetooth-standard's phone-call protocol. That is, I don't think there's an app that can just monitor the microphone.
If you want to listen to the people sitting in a room with your cell phone from your Onyx, there are several PA "microphone" apps that let you use your phone's built-in microphone to broadcast to a Bluetooth speaker. I tried a few of them and found "Microphone
" (by Wonder Grace) to be the best. It works even when the screen is off so it could be used to inconspicuously monitor a room without draining the battery on the screen.