Choosing the appropriate sound and music
Choosing the sound and music will likely be one of the last things you do for your game (unless, of course, you're making a music game).
It is important that the music fits with the tone that you have set with the art and overall feel of the game. For me, this is harder than choosing art, because I know that choosing the wrong music could drastically bring down the appeal of my game. If the player finds your music choice annoying, too loud, or overbearing, their experience will be far from ideal. I recommend playing the music you chose over and over for yourself. If it annoys you after a few loops, it will definitely annoy the player.
Appropriate sound effects are also important. You want to make sure that all appropriate actions have sound and that the sound fits the action that it accompanies. You don't want the sound of breaking glass to accompany the action of a character sighing, for example. Too few sound effects can make your game feel incomplete...
