I am assuming that you're aware of the difference between an analog signal and a digital one.
Anytime you record an analog signal on a digital machine, the signal must have passed through an "A/D converter", which is an analog-to-digital converter. What the converter does is sample the incoming analog voltage fluctuations at a specified rate (the sample rate!) and record its measurements as binary data (samples). Thus, your nice continuous analog waveform is chopped up into little discrete pieces that are supposedly close enough together that you can't hear the difference....kind of like how a movie is just 24 still frames running past your eyes every second.
In order for you to actually hear a digital "signal", it must first pass through a "D/A converter", or digital-to-analog converter. This creates a new continuous analog voltage wave from the discrete digital sample values.
Obviously, when you're having electronics chop things up and measure them, then later try to reassemble them...things are going to get a little ugly. For one thing, the clock used to sample the incoming audio and to render the outgoing analog audio must be as precise as possible. If it's just sampling when it feels like it, it's going to be awful hard to put back a waveform that looks as close to the original as possible! This time-based distortion is known as jitter. You also need to consider sample accuracy. If the converter cannot accurately measure or render the audio, it's not going to sound very good!
So, long story short, whenever you convert from analog to digital or digital to analog, you're doing some amount of damage to your sound...even on the best systems...it's just simple math! The better your converters, the less *audible* damage.
So, if you take an analog line out of your digital recorder into the analog line in on your soundcard, you just converted the signal from digital to analog and then from analog to digital...you just passed it through two conversions and it may not end up sounding as good as you want.
This has nothing to do with bit depth or sample rate, really.
Slackmaster 2000