While some songs sound just fine as instrumentals, many more could benefit from the addition of a vocal sample or two.
Predictably enough, SampleRadar can help you out on this particular score: we've got a selection of short vocal sounds that you can use individually, stitch together or process into oblivion.

What you need to know
The vocal ad-lib samples are divided into three folders containing ad-lib, FX and spoken word samples.
All the samples are supplied as 24-bit WAV files so can be imported directly into your DAW or sampler of choice. Because they're royalty-free, you're welcome to use the samples in your music in any way you like - all we ask is that you don't re-distribute them.
The vocal ad-lib samples are supplied in a zip file, so you'll need to extract them before you can see them. Enjoy!
Example sounds
Ad lib 140bpm
Ad lib 103bpm
FX 170bpm
Good morning
Vocal ad-lib samples: click to download
Vocal ad-lib samples (237MB)
These samples originally appeared on Computer Music magazine's cover DVD. Check out the latest issue for many more.
Tutorial
How to produce vocals