Bad audio quality from two stage audio amplifier
The 47 &micro;F cap (C3) in series with a 8 &Omega; speaker forms a high pass filter with a rolloff off 420 Hz.  That's the point at which the volume is already down by 2, and it gets lower inversely proportional to frequency below that.  The reason "instruments" sound better than voice is because voice has more frequencies below this point.

However, that's only one problem of many with this circuit:

Q1 will drive the output up with lower impedance than R5 will drive it down.  The result will be assymetrical output shape to a symmetrical input.  In other words, distortion.

There is no cap across the power supply.  When the output is driven high, it takes more current, which sags the supply.  This feeds back into the first stage making a mess, possibly even causing oscillations depending on the impedance of the battery.

There should be a decent size cap across the power supply, and the supply to the early stages filtered to protect them from noise on the supply anyway.

2N2222 is inappropriate for driving a low-impedance load like a 8 &Omega; speaker.  If you fixed the output drive, then the next problem would probably be that Q1 is getting fried.

The DC bias is quite dependent on the specific supply voltage.  It looks like it will be close enough for the 10 V supply shown.  You want the output of Q2 to be about in the middle of its possible range.

C1 causes a high-pass rolloff of about 480 Hz.  Below that, the gain is roughly 10.  Above that it goes up with frequency.  For audio, you want a flat gain across the intended frequency range.  That frequency range is 20 Hz to 20 kHz for "HiFi" audio.

This is now the second overall high pass filter, both together making "voice" sound a lot worse than "instrument".



Overall, if you really want to drive a speaker, ditch this mess and use a circuit intended for that.  Driving a speaker isn't as simple as slapping a cap on the output of a low power amp, like what seems to have been done here.