First, read this question:
- How are sound effects made?
Second, there are so many musicians and sound designers of all skill levels (and expecting various degrees of compensation from free to standard rates) wanting to join on with a project. If you’re having trouble finding somebody, you just need to find more internets. ModDB, GameDev.net, forums for specific engines/tools that have tried to form a general game development community. Seriously consider giving working with somebody a shot. You’ll both learn a lot.
If you’re wanting to be a one-man developer, then you can take a sound editor, microphone, and samples and synths, and now you’re a sound guy. It’s another art/craft to learn, no different in that regard to drawing/modeling/other-graphic-art, game design, and programming. Use the tools Valryon mentioned (SFXR and Audacity) to get started. Also look into:
- Wavosaur – A freeware sound editor
- Reaper – A very inexpensive DAW (digital audio workstation, used for producing music from audio recordings and midi) and Ardour, a FOSS alternative for Linux and OS X
- KVRAudio.com for free basic VSTs (often playable directly in Reaper and other DAWs
- Modplug, Renoise, SunVox, and Buzz are music trackers for old-school-style game music composition
- Links for the lazy: Audacity and SFXR
For learning about sound, you should be aware of some of its physical properties and how we are able to digitally represent it.
To start making music, you should learn about music theory and how to produce music.
There are many sound banks on the Internet providing sound effects or musics for games. It can be free or not, in all formats and quality you want. An example: Findsounds
You can mix those samples, or modify them to create new sounds. A fridge sound can be transformed in a spaceship laser sfx. Some softwares are quite easy to use to do that, such as Audacity.
If you are into retro/chiptune sound effects, there is an awesome software to make those kind of sfx : SFXR. You will spend some hours getting what you want but you can make everything with it.
Another solution is to record on your own. You need a mike and a lot of imagination (like this bread landing in the bath producing an octopuss being scratched sound…). But results may not be terrible.
Finally you can do all your sound effects with your mouth, the result may be.. unexpected!
As a one-man developer, I’ve got something quite interesting – I invested a few months into writing a piece of software which autonomously generates music. This doesn’t work for everyone, of course, but it is really, really, helpful, since some music in the style of the game really helps me get into developing. If you’re not a musician, then consider looking at other algorithmic composition programs.