Hello! I had this issue with some emulators, not only with SNES. You can solve it modifying this file:
/opt/retropie/configs/all/retroarch.cfg
Search for this lines and uncomment/modify them:
video_fullscreen = true
video_force_aspect = true
video_aspect_ratio = 1.77
This is for the video, for the sound you must modify this one:
audio_out_rate = 44100
