How to split up voltages
simulate this circuit &ndash; Schematic created using CircuitLab

You simply cannot use V=IR for this problem. The current these modules take varies every time. So the I is not constant and the required R will vary every time. 

What you should do is using the batteries in parallel so they will give you 3.7V output but an increased capacity - 8600mAh. Then use those two parallel batteries to power the both devices in parallel. 

And by the way, PAM8403 works well with a power supply until 5V. I've used it myself.

P.S. - Ignore the OP Amp symbols I've used to the draw the schematic. They are your Amp and BT Audio module.