While developing I like to create small temporary sound effects and implement them in code, this way all I need to do is replace the asset with the real sound effect, and I don't have to change my code at a later date. This saves me time, and also allows me to see where sound effects are needed very early on so I can add them to the asset list. Coming up with these sound effects can be somewhat of a pain if you don't know where to start. Do you find free sound effects on the internet? Do you find a sound effect generator? Do you make fart noises with your mouth?
I ended up going with the sound effect generator method (though making fart noises was a close second), and came across two programs that were closely related to each other. Sfxr, and Bfxr. Both of these programs are really good for creating short random tempory audio files, or creating 8 bit and 16 bit audio files for your games requiring that retro feel.
I came across Sfxr a few years ago while I was in school. It is actually what I used for the sound effects in my old tutorials on how to create a game in Unity. Sfxr is a free tool created by someone who goes by DrPetter, and it was created back in 2007 for the Ludum Dare challenge so people could create basic sound effects for the games they needed to complete in 48 hours. Sfxr has it's source code available for anyone to fiddle with, and you can use any sound effects you make completely free.
The interface is very basic looking, but if you are creating basic sound effects you don't need anything flashy. Here is an example of what the program looks like:
If you are looking for something extremely basic, Sfxr is the way to go. You can grab it here: http://www.drpetter.se/project_sfxr.html
Bfxr is something I just came across recently while doing my usual random internet searches. It is an extremely heavily modified version of Sfxr. It added a number of new options and values to tweak, which for some may be too much to handle at first glance. In addition Bfxr saves old mixes and sounds in a list for you to reference at any point which is a huge improvement over Sfxr, which requires you to save it to the disc if you ever want to load up the sound again. Bfxr is great if you want to have more variety and more control than what Sfxr gives you, and you aren't afraid of an interface with a large number of sliders and buttons. Here is what the interface looks like.
Bfxr is also free to use and it's source code is also hosted on GitHub so anyone can make a pull request and fiddle around with it. You can grab it here: http://www.bfxr.net/
So whether you are looking for placeholders or are looking for some retro sounding effects, either of these tools will be very useful to you. Both offer a ton of options and are free to use. Anyways, I'm off for now, and if you know anyone that is looking for sound generators, you know where to point them.