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I have a couple additions to the excellent suggestions so far:
For 8-bit retro style sound effects try sfxr. It’s fun to play with even if you don’t need it. (There is a Mac version lower on that page.)
If you are on a tight budget for a small project, try making your own sound effects and music.
Using the built-in microphone on a laptop works in a bind. Use a tool like Audacity to mess with it a little and you might get what you need.
Music software in general has come a long way towards the novice ever since GarageBand was released. Depending on your project needs, you might be able to put something passable together using an inexpensive music sequencer.
Of course, you are going to get your best results from professionals.