900 MHz audio transmitter and receiver

I'd like to buy a 900 MHz audio transmitter and receiver to connect two home stereo systems. I want low frequency for better obstacle penetration. (They are separated by only 20 feet, but there’s a chimney in between and either that and/or interference has created problems with 5.8 and 2.4 GHz senders/receivers.)  Does anyone have any suggestions.  I only need audio transmission, but I'm assuming that AV is more readily available, but I can find and at 900 MHz.  Is there some other option?  (Bluetooth won't work since I already use that in the same system, and Bluetooth cannot transmit to two reception points, as I understand it....)