I agree with S&R. Hiss is nearly always a result of a low signal to noise ratio. Every electrical component- even a length of wire- generates noise. However, if your signal is many hundreds of times louder, you won't hear it.
One of the first things you need to learn in sound recording is called "gain staging". Briefly, it means that you must ensure that the signal going into your recorder is at a level so that the loudest sounds reach the maximum level that your equipment can record without distortion. That is the definition (in recording) of Zero dB.
Are you "playing it safe" by running lower levels. If you are recording to, say -18dB, your signal is only one eighth of what it could be.
When you turn up the playback to get back to normal levels, you will have boosted the noise eight times too.
You don't say what your recording medium is- is it intrinsically noisy- like an ancient Soundblaster soundcard or a cassette recorder for example?
The second thing- what mic are you using?
Muddy guitar sound is caused by:-
a muddy guitar
dead strings
an unsuitable mic
bad mic positioning
We need more exact symptoms before we can make a successful diagnosis.
Cheers
Dave