If you ever want to edit audio on Windows, the free GPL-licensed WaveShop would come up top on the list.
But it is possible (at least for MP3s) in a somewhat obscure way: changing the sampling rate.
I'm not an expert on acoustics, music or MP3; but a quick googling shows that the sampling rate is the number of samples of audio carried per second.
Thus, if you reduce your sampling rate to 80% (say 44100Hz → 35280Hz), it would make your MP3 player think that the samples in your file were taken at 28.34-microsecond intervals instead of 22.67 - a 25% "stretch".
So your track will actually play 20% slower, taking 2 min 5 sec if it was originally 1 min 40 sec. Exactly what we need!
To use WaveShop to change your audio track's tempo in this manner:
- open your track in WaveShop.
- go to Audio → Format....
- based on the tempo change you need (say x%), enter the correct Sample Rate value; note that if you want to slow down, x would be negative!
current sampling rate × (x/100 + 1)
- click OK.