To convert a WAV to MP4, open the WAV above, add the artwork, switch off the volume levelling so your master is left as it is, and export. The result is an MP4 with your artwork as the picture and the song as 192 kbps AAC sound, which YouTube, Instagram, clients’ phones and every media player will open.
WAV files usually come from a studio, a DAW or a mastering engineer, which changes what matters. You want the mix to sound exactly as approved, the start and end untouched, and a file that isn’t ten times bigger than it needs to be. This page opens the studio on the Audio panel for that reason. FLAC masters work the same way. For an ordinary MP3, the MP3 to MP4 converter has the general steps.