The BTD-500 requires the installation of firmware in order for audio and other features to function. The driver is present in the Linux kernel but the firmware is not. The firmware is not open source and is not included with Linux distributions.
The firmware is available at the link below:
Open a terminal window, then extract the files from the ZIP file with the unzip command.
Copy the files with the following commands:
sudo cp rtl8761b_config /usr/lib/firmware/rtl_bt/rtl8761b_config.bin sudo cp rtl8761b_fw /usr/lib/firmware/rtl_bt/rtl8761b_fw.bin
Next, power off the computer, plug in the BTD-500, then power the computer back on. The BTD-500 should now operate as expected.