Keyborg,
ah, that is the art part of the science, ain't it? Everything, nothing. It depends upon how you hear your music. Vocals almost always need compression to make them "sit" at an even volume in a mix - bringing up the lower (volume, not bass) parts while the louder ones don't overwhelm the music. Compression narrows the dynamic range of a track by keeping the volume from jumping over the threshold (maxium volume) too quickly.
A lot of lead instruments need it too. and anything that is overly bouncy in volume if you want to hear it in a mix, such as bass.
EQ is used to carve out a sonic space for an instrument in relation to the rest of the instruments. Normally, you would cut the low end of higher registered instruments to leave the low frequency range to the bass and kick. Vocals often are cut in the lower registers, since you often don't get anything down below 100 kHz except for noise and rumble. Even Barry White doesn't have much good info down low. I've used a high pass filter up to 250 kHz on female voices. It is usually better to cut out frequencies than boost them, since boosting can more easily cause problems w/ phase etc. You want to find the frequencies in a sound or instrument that define the sound, and either boost it or, better, cut out those frequencies in other instruments to give it space to be distinguished.
Of course, if you have to wonder whether an instrument needs comp or eq it probably doesn't. It is easy to overdue effects just because you can. Until you learn what works for you, play w/ comp and eq, just be aware you can cause more problems than you solve. Generally (again! and like reverb), one should augement the effect until you can hear it plainly, then back off a bit. If you can, disable the effect while listening to see how it works both in and out.
Finally, many audio loops (and midi for volume) are already "pre-processed" w/ eq and compression.
Though there are many standard practices for mixing, tho aren't a religion. As you find those "tricks" which work for you, you are developing your "sound."
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