if two modules talk to each other and they need to understand each other they need to speak the same language. Same baudrate and same protocol.
If you have 3 nodes in a network { A, B, C } that means that you have 3 possible pairs {A,B} , {B, C}, {C, A} ..
Any pair need to talk at the same speed, but different pairs don't need to. In fact often one wants to maximize the baudrate per pair .
in you case
PC ---(1) ----- MASTER ----(2)---- SLAVE
(1) hardware serial - The PC and the master could talk at 115200 baud, while
(2) the BT connection - could be done at 9600 e.g. because that is the max baudrate at a given distance.