How to drive a speaker/microphone from a digital circuit?
The speaker moves both ways, this is why you want it to rest at 0V.  Otherwise you are only ever pushing on the speaker and never pulling on it (or vice versa).  This works, but you are not using your speaker optimally.

The series capacitor with resistor to ground form a high pass filter (which blocks DC and idles at 0V).  Now you can DC bias your signal so it idles at 2.5V.  The high pass filter blocks the DC, so on the speaker side of the filter it is idling at 0V.  This way your signal both pushes and pulls on the speaker.

To answer if 5V is enough to drive your speaker, we would first need to know how much power you are expecting.  To find the RMS value of your 0-5V signal:
Vrms=0.707∗5V=3.5V

And since power is voltage squared over resistance:
(3.5V * 3.5V) / 8Ω = 1.53W

Your question about adding two sources should be a separate question.