App note from Microchip about wireless multi-speaker connectivity solution. Link here (PDF)
Microchip’s Multi-Speaker Bluetooth Audio solution utilizes Microchip proprietary technology to connect a master speaker to one or multiple slave speakers through a modified Bluetooth protocol. Multi-speaker functionality is implemented using Microchip’s BM6x family of modules. One (master) speaker can either be connected to an audio streaming Bluetooth device (smartphone, tablet, laptop, etc.) or its own auxiliary input (AUX/LINE-In jack), and re-transmit either of the two audio sources to one or more speakers acting as a slave.