Trying to down-sample audio
First you need to interpolate between samples rather than just retaining and discarding samples (which introduces horrible jitter noise during non-integer downsampling).

Then, if the original audio data contained spectrum around or above 4000 Hz (half the sample rate), you will need to low-pass filter before or in conjunction with the interpolation to the lower sample rate, or else you will get aliasing.

A good combined method of low-pass filtering plus interpolation is to use a windowed Sinc function as the interpolation kernel, where the Sinc is scaled to be the transform of an ideal "brick-wall" or rectangular filter with the appropriate low-pass cut-off frequency.  The width of the window on the scaled Sinc function will control the transition width of the frequency cut-off.