Sounds are a very important part of a game. Without sounds, people will get bored of your game very quickly. Their is mainly two types of sounds that you will want to use in your game. Those sounds are sound effects and music.
Sound effects can do a lot for your game. Think of it this way. If a character is repeatedly slashing a sword, you will want the sound to be something that the player can bear to listen to over and over again. So choose an effect wisely. Some good places to find sound effects are SoundSnap, FindSounds, and FlashKit.
Music is also a vital part of a game. Whether it is ambient music in the background of a side scroller or rock music for a guitar hero type game, music could make or break your game. If there is a song playing throughout the whole game you will want it to be something that isn’t to distracting, but also has a nice rythem to it. Make sure you add a mute button, so people who want to play their own music can. A good place to find music is Newgrounds Audio Portal.