Do you care if the pitch changes?

No: Then you want time stretching.

This is simply interpolating / combining samples to generate the same overall waveform in a different number of samples. The obvious nearest neighbor approach will cause severe aliasing but technically works. To make it sound better one can use a higher order function like sinc interpolation.

Yes: Then you want time scaling.

This is usually boiling down the sound to a procedural / time domain representation, it is an active area of research and doesn't sound perfect. The common techniques are:

1. Phase Vocoder
2. Time Domain Harmonic Scaling

EDIT: Your question is WAY out of scope for SDL. If you just want a library (instead of understanding what you are doing) then try SoundTouch or Rubber Band.

Do you want the pitch to warp with the speed?

Yes: Then you want time stretching.

This is simply interpolating / combining samples to generate the same overall waveform in a different number of samples. The obvious nearest neighbor approach will cause severe aliasing but technically works. To make it sound better one can use a higher order function like sinc interpolation.

No: Then you want time scaling.

This is usually boiling down the sound to a procedural / time domain representation, it is an active area of research and doesn't sound perfect. The common techniques are:

1. Phase Vocoder
2. Time Domain Harmonic Scaling

EDIT: Your question is WAY out of scope for SDL. If you just want a library (instead of understanding what you are doing) then try SoundTouch or Rubber Band.

Do you care if the pitch changes?

No: Then you want time stretching.

This is simply interpolating / combining samples to generate the same overall waveform in a different number of samples. The obvious nearest neighbor approach will cause severe aliasing but technically works. To make it sound better one can use a higher order function like sinc interpolation.

Yes: Then you want time scaling.

This is usually boiling down the sound to a procedural / time domain representation, it is an active area of research and doesn't sound perfect. The common techniques are:

1. Phase Vocoder
2. Time Domain Harmonic Scaling

Do you care if the pitch changes?

No: Then you want time stretching.

This is simply interpolating / combining samples to generate the same overall waveform in a different number of samples. The obvious nearest neighbor approach will cause severe aliasing but technically works. To make it sound better one can use a higher order function like sinc interpolation.

Yes: Then you want time scaling.

This is usually boiling down the sound to a procedural / time domain representation, it is an active area of research and doesn't sound perfect. The common techniques are:

1. Phase Vocoder
2. Time Domain Harmonic Scaling

EDIT: Your question is WAY out of scope for SDL. If you just want a library (instead of understanding what you are doing) then try SoundTouch or Rubber Band.