How do I improve the quality of sound when I send a picture message?

You seem to be confusing technologies. "Grainy" is not a descriptor of sound files. Increasing the quality of sound has nothing to do with "picture" messages (except in television), nor will you save a picture as a .wav files, since .wav files are audio files. If a website limits the size of audio files that can be uploaded, you have no control over the audio quality, nor have you any control over sampling rate, which is irrelevant in this case.