Recording and playback of sound, frequencies question
If your sampling frequency is $f_s=8000$ Hz, your maximum signal frequency is indeed 4000 Hz ($=f_s/2$). If your signal contains frequencies above $=f_s/2$ you would hear the results of aliasing. This means that the original spectrum is folded back into the range $[0,4000]$ Hz. What actually happens is that by sampling with a sampling frequency $f_s$ your spectrum is made periodic with period $f_s$. So if the signal is not band-limited with maximum frequency $f_s/2$, the resulting periodic spectrum will have regions of overlap. E.g., if your signal has frequency components up to 5000 Hz, these frequencies would get folded back to the frequency band between 3000 and 4000 Hz, and consequently lead to distortions in this band. This is the reason why sampling is always preceded by lowpass filtering to attenuate all frequency components that might lead to aliasing (anti-aliasing filter). You can listen to some examples of aliasing here.