Yikes!!! What a barrel of monkeys you have opened here. As I always like to say, "If you open a barrel of monkeys, be prepared for a gorilla to jump out at ya".

But since we are talking digital audio here, my recent trip to the zoo really doesn't have much pertinence now does it? Or maybe....
Anyway. drstrawl has given you half the information concerning what we can hear, and what we can't. Actually drstrawl, the highest frequency a human was ever tested hearing was about 18Khz. Next, just because you can't hear it doesn't mean that it doesn't have a major effect on the sound. Trumpets create frequencies up around 28-30Khz range. You may be thinking to yourself that that is almost a full octave (double the frequency) higher than you can hear, so who cares? Well, physics care!
You see, it is the upper harmonics, or the overtones of an instrument that give that instrument it TIMBRE, and I don't mean lumber or cute trees here, I am talking the actually perceived sound of the instrument. Have you ever asked yourself why two instument that are seemingly exactly the same, playing the same note sound quite a bit different. You can blame it on the player, but there are things called Phycoacoustics in play here. Now I am not suggesting that sound is crazy (well, on second thought......)phycoacoustics have to do with perceived sound, and what has an affect on it.
On we go here. The reason we concern ourself with upper freqs is because they give instruments their very distinct sound. If you can't capture those freqs, then the accuracy of the sound is diminished. In the case that you can't hear those freqs, it doesn't matter because it more has to do with the way that those freqs mess up what you can hear. So, we need to have these freqs present to mess up the sound for us so that everything sounds right. Almost doesn't make sense to have something messing everything up and calling it the right thing to do, but that is the case here. Without those little weak upper freqs clashing around, the perceived sound is less detailed.
Now things get tricky. You may be asking yourself why most audio components only go up to 20Khz if you need the stuff much higher than that. Of course you could also ask the logical question of why a recorder would need the ability to record higher than your ears can hear. And these are very good questions. But, if you think like a microphone and a speaker, you will get much better result in the studio.
Here is the deal. If somehow you could just put yourself inside of your digital recorder and hear what is there, man, that would be some sound. Unfortunately, we can't do that so we rely upon speakers to do the exact opposit of what a microphone does. In neither case do they do this as well as the ear. The ear is one sensitive microphone. And the brain is the ultimate speaker. This must sound crazy, but really, I am not high on anything, just using some color for some very dry stuff here.
So, since a microphone has to pick this stuff up, and a speaker has to reproduce it, and since neither is as good as the ear, we need to overkill a few things here to help out. Actually, most audio equipment does have a frequency response that is quite large. Manufactures only concern themselves with telling you how well they respond to frequencies that you CAN hear. But when you start reading up on the phase shifting of the stuff you can hear, that is where you can make pretty intelligent choices about the equipment you purchase.
So let use the 44.1Khz sampling rate here. It is a fact that a you can only record a frequency half the sampling rate. Soooooo, if the music contains information that is much higher than this, well, you are missing out on the stuff that gives it realism. It is as simple as that. The reason that sampling rates on professional recorders is standard at 48Khz is because is sounds better. More of the detail in the sound is picked up and reproduced accurately. So in the case of 96Khz, we would definately be able to record ANYTHING that might affect the perceived sound. That is why things are moving this way. So, if you have the hard drive space to record at this sampling rate, by god, go for it. You definately will hear the difference. As far as the material getting downward sampled to 44.1, well, you are still going to wind up with a better sound on the CD.
Next. Bits o' resolution really have to do with variances in the volume of the frequencies. More bits gives more procession power to dedicate to reproducting tiny fluctuations of volume that were picked up by the converters. More information can be stored on tape. So, go with the highest you have if you have the hard drive space.
Maybe play around with say a 88.2Khz, 20 bit recording for the source tracks. This will give you much better fidelity than is currently available on many widely used recording formats. That is, if it is true linear 20 bit. Watch out here, many products don't deliver true 20, or 24 bits of resolution. They use some data compression schemes to simulate it.
Next. A compressor on vocals can actually lead to high lighting an already bad sound by making it more present in the mix. If your vocals sound like they are coming from a Folgers can, well, first I would ask why with all the Starbucks in the world, you are still drinking that crap....

Next I would suggest that maybe you are not using a very good microphone or microphone pre-amp, or that possibly you have some cable somewhere in the signal path that is bad. Compression will not alter the tone of anything severly unless used in excess. So, try borrowing a really good large diaphram condenser mic, and a decent outboard mic pre-amp for your vocals, and then see if that will improve things. Also, when mixing a vocal, about the only thing you will need to do with it on the EQ is to roll off some low frequency if a high pass filter wasn't employed while you recorded the track. MAKE SURE THAT YOU ARE GETTING THE RIGHT KIND OF TONE AND PRESENCE ON THE VOCAL WHILE YOU ARE RECORDING!!! Any EQ'ing on a vocal is usually to it's detriment.
Hope this helps. Good luck.
Ed Rei
Echo Star Studio
www.echostarstudio.com