That's whats great about audio - so many ways to skin a sound.  Traditionally, this was mastering, which was all about matching tape to the limitations of vinyl.  Nowdays it means getting the hottest possible levels to CD.  A good mastering engineer knows how to do this and many are reasonable with their rates.  However, you can try yourself and might find that you can do a good job, as good as many who advertise themselves as mastering engineers. Different tricks, but the principles are the same in audio.

If the songs are all over the place (and a good mix of a ballad propably ain't gonna sound as loud as hard rock one) you need to establish which songs need to be up and which one(s) is loud enough.  Good suggestions above, but something like Sony's CDArch is good for a linear look (and hear by expanding the view and skipping around from song to song).

Next is to raise the preceived levels close to one another across the CD/EP.  To raise this I have several tools - voxengo's elephant/curve EQ, TC Electronics DSP EQ and compressor and external hardware (komit comp/limiter).  I use the komit for the heavy lifting - it has a good "clean" ssl type of compressing followed by a analog brick wallish limiter that goes from saturation to distortion.  I use a digital comp/limiter to lower any outrageous audio spikes, eq to trim any energy blooming spectrums and the komit compressor to raise the overall level of the trimmed sound.  The analog limiter is a backstop, letting me get a final bit of compression and then use the output gain from the comp to bump up to the signal level set by the limiter.  If I push it hard it sounds good on rock - pushing to heavy saturation/distortion not so good ballads.  Sometimes it will take a few tries recording the signal back into digital to get it just about right - or as right as I can get it.  More than a few times I've pushed too hard and get too much saturation upon loud listening later.  And you need to reassemble these files into your CD project to let them rub against one another.  Even then, there will be some different vol levels, but in CD architect you can use the vol envelope to further even out volume levels between (or within) songs.  It only goes to 1 dB (in .1 increments) so if you need more than this it is back to mastering.

Home mastering can be done - SONAR has the tools although specific tools really help - Sound Forge, CDA, mastering software and high-end analog help lower the difference in quality between semi and pro mastering.  But even more important is the skills - it takes a lot of screwing up before you understand what to do and what not to do.

In your situation, if you think your produced stuff stands up to pro levels give it a whirl.  See what you think.  Then check w/ the clients.  You want them happy even if you need to go outside while you work on your mastering chops.  It ain't rocket science, but it takes a different ear and skill set than recording or mixing.

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