What is "treble? "muddy"?

Many of those terms are relative, but a good way to look at it would be the notes on a piano. The notes on the treble clef line on sheet music are higher, and the bass clef ones are lower. Other than that, high strings and especially female vocals are part of the 'treble' range.

Upper midrange/lower treble can be heard throughout this song and in the chorus:

Here is what would be called upper treble as it pertains to female vocals, listen starting at 3:50:

As for muddyness, I think it's more understandable if you can get your hands on an open pair of bright (shiny clear treble) open-type headphones, and compare them to something that has a closed back (no vents or grille on the headphone) and lots of bass. One sounds like the bass is tight, and the other has more bass, but it's less clear and sounds a bit more muffled usually.

Fundamentals are the notes that a singer/instrument is making the most of. When you sing a note, the fundamental is most of what you hear, but there are other frequencies that are coming out at the same time along with the fundamental. These are the harmonics, and certain good harmonics and warmth and body, or fullness/texture to a note.

That chart was really excellent.

You can have muddy bass, and muddy middle. Who knows where one starts and ends?

Very crunchy topic...

If you have a music player (software or hardware) with a decent EQ, that can help you understand what treble is and does. Try listening to a familiar song that has a good variety of instruments, then turn the treble frequencies all the way down and listen to the difference. Play around with turning up/down different frequencies and you'll be able to hear what they are contributing to the mix.

Thanks for the chart, very interesting.

I think its possibel that these terms can  vary between different people, for example I listen to a lot of motown/pop and my definition of "muddy" I am sure will be different to someone who listens to death metal!  With that said;

Muddy to me is when the instruments are locked in together and often have a groove, I guess in my mind & ears its when its a bit more bass heavy.  

Compared to "clean" sounding which to me is when I can differentiate between all the instruments and it sounds bright.

This my CLEAN example

http://www.youtube.com/watch?v=F-mjl63e0ms

This is my MUDDY example

http://www.youtube.com/watch?v=pCUD52feOmg

Not perfect examples, I guess ideally I need to find the same song, but 2 different versions.