Can you bridge or parallel the outputs of the PAM8403 amplifier?
here's the response I got after emailing Diodes:


  We would advise NOT to connect the two outputs of PAM8403 in parallel.
  We recommend two ways to get higher power :
  
  a ) increasing the supply voltage, from 5V to 9V for example---PAM8320
  working at 4.5V to 15V is a good candidate for this solution. 
  
  b)  reduce the loading impedance, from 4Ohm to 3Ohm for
  example---PAM8406 can drive as low as 2.5Ohm/Ch that make PAM8406 is a
  good choice.


Conclusion:

You cannot bridge the PAM8403 (as you can't bridge two pairs of differential outputs), and Diodes have recommended against using the two outputs in parallel.