connecting pam8403 and bluethooth module in series
No.  You cannot connect the two items in series.

First of all, 5V+5V is less than 12V - you are still two volts short.

Second, the voltage will not divide evenly across the two. Consider them as a voltage divider:







simulate this circuit &ndash; Schematic created using CircuitLab

If the two devices needed exactly the same current to operate, then they would have the same resistance and would divide the voltage equally.

It is very certain, though, that they do not need exactly the same current to operate.  The voltage across each one will be different.  Since they are active devices, the current each consumes will change continuously - the voltage across each will therefore also change continuously.

Even it it did divide evenly, you still have the problem that you must connect the ground of both devices together.  That puts a short circuit in to your voltage divider.

So what do you do?

Use a regulator to reduce the 12V down to 5V.

There are switching modules that are easy to use, and linear regulators that are cheaper though more wasteful of energy.