I found an audio circuit and I built it just fine, but I find it a bit too quiet. How do I amplify the output so that it is a bit louder?
Actually, the peak-to-peak voltage is going to be close to 7.4 V. This is because the speaker is being used in a differential mode, connected to two separate MCU outputs, with neither terminal grounded.

Since the firmware is simply driving the two pins with complementary waveforms, you can use just one of them (with a suitable filter, as JRE shows) to drive an external amplifier.

Of course, the speaker will also get somewhat louder if you simply raise the supply voltage to the full 5.5V that the ATtiny85 allows.

However, the primary limitation is not voltage, but rather the current capability of the MCU pins &mdash; &plusmn;20 mA nominal max, &plusmn;40 mA absolute max. If you were to simply add external buffers to those pins, you'd get a lot more sound out of the speaker:







simulate this circuit &ndash; Schematic created using CircuitLab

Note that the buffers are configured as emitter-followers. There's no voltage gain here (in fact, it loses about 1.3V in peak voltage), but a lot more current is available to the speaker. Since it's a digital signal, we don't need to worry about the horrible crossover distortion of this configuration. But it should be able to deliver about 4.8 VPP @ several hundred mA (1.7 VRMS @ 200 mARMS into 8 &Omega;), or about 360 mW of audio power!

(BTW, this is the solution brhans was alluding to in his very first comment.)