Treble sparkle would be the depth and detail that come along with forward sounding highs that occur in the 5K and higher ranges of the frequency spectrum. That can be fatiguing to some, other love it (like fans of etymotics). This headphone (as with most of Sony's audio offerings and "house signature") have a roll off in the highs starting at about the 6K region and upward. This means that the very high frequency sounds are generally going to be less audible than other parts of the frequency spectrum. This is going to impact the sound of a lot of percussion instruments, ambient sounds, and other sounds that generate high frequency. It will make them sound less sharp. I like the way Sony has done it because the detail isn't lost in these regions at all, but it takes away some of the sharpness of these sounds and makes it more comfortable for longer listening sessions.
I'll be honest, there are times where I hear things people say and it sounds like they are just making things up. Really what it is is a way of describing how the frequency responses impact how they sound to people. If you ever watch any of the recent videos from lachlanlikesathing on youtube, you'll hear him use a lot of terms like this. As you get to experience a lot of different headphones and sound signatures, you'll get a feeling for how to describe how things sound to you. The jargon will start to make sense.
Welcome to the wonderful world of head-fi!
I was lost until you said "less audible", then you lost me again. does something sounding "forward" mean that it sounds like it is in front of you? like when a band is in front of you, that is where the sound is coming from? I know I will get hate for saying this, but it is hard to communicate with people around here sometimes, because all you get to do is read their words, and you can't hear their tone of voice, and the words come across as just so pompous. I am not meaning you, I am speaking in general terms.
in college, one of my engineering professors told us "you are going to be engineers, but most of the people you are going to speak with, will not be engineers, so learn to take what you know, and talk about it in a way that non-engineers can understand." I wish there was a way I could say that to the audiophile world, without sounding like an *******.