How to resample in MATLAB one signal that is supposed to be convolve with another one?
If the ratio of the sample rates is reasonably rational (i.e can be represented as the quotient of reasonably small integers) then you can use the MATLAB function resample().

Convolving to audio signals is rather unusual. Typically and audio signal is convolved with filter impulse response. The impulse response could be stored in an aduio file, but that would be a bit unusual. I would double check if you understood the task correctly.

Both interp1() and spline() are poor choices for sample rate conversion as they don't control the mirror images in any predictable way. I recommend reading up on "sample rate conversion".