Phoebe,
An audio power amplifier has a voltage loss. Use an imaginary amplifier that is perfect with NO voltage loss and use a 9.0V power supply, not a little 9V battery (that becomes 6V soon).
Then the maximum voltage swing into a speaker is 9V peak-to-peak. 9V peak-to-peak is 3.18V RMS. Then the maximum power into an 8 ohm speaker is 3.18V squared/8 ohms= 1.26W.
You will be lucky to get 0.5W from a real amplifier.
It will sound slightly louder than a cheap clock radio.
I bought a new clock radio for $1.75 a couple of years ago and it is LOUD but it sounds awful! I disconnected its cheap tiny Chinese speaker and connected a good little hi-fi speaker system instead then I increased the value of its output coupling capacitor to 4 times and now it sounds excellent.