I'll reply to Mario then respond to NA's comments:
Mario:
Basically, my understanding was that meditation was putting attention to sensations, no matter what they were, so I thought "Hey.. if that's the case I can do that with music... let's do it! "... and so I kept doing it for a while, being the only kind of meditation I did, for something like 1 to 4 hours every day.
There's no problem with that, just make sure you're applying clear and precise attention to each sensation without getting pulled into mental stories
about those sensations.
Mario:
Thinking about it, it seems to me that I progressed through the vipassana jhanas in quite a natural way; however I'm not completely sure, so I post this here for comments, precisations or anything else from others.
Your descriptions suggest that you probably did encounter at least 1st and 2nd jhana, and possibly the early stages of 3rd. My advice to you would be not to get hung up on a diagnosis so long after the experience, there's always an element of deletion and selective remembering involved (unconsciously, not that you've deliberately manipulated the memory to suit or whatever) when going back over this stuff and
looking for correlations.
Mario:
I also remember that, a few times, I experienced a very strong shift, and attention would become very wide -but not in all of the body, just wide related to sounds- and every single sound was clear and experienced at the same time, and it was kind of like if there was this big space inside my head that would encompass every note... obviously, I think to this as the fourth vipassana jhana. (and it was absolutely wonderful, by the way).
I'd suggest 2nd jhana, purely because I have no idea what you were doing prior to this and what other sorts of sensations you noticed. What you describe, the shift and the wideness/internal spaciousness, are things I'd associate with 2nd. Bear in mind too that 2nd and 4th share certain characteristics to some extent, and this can make it really easy to get mixed up if you're not familiar with the ins and outs of the model; 4th vipassana jhana is quite distinctive in it's clarity and spaciousness, which is of a different and more volumetric nature than what you get in 2nd, but it's incredibly normal feeling without the sort of spatial fluctuations you mention.
All in all, it sounds like you're getting into the right territory but I'd recommend refining your technique a bit, dropping expectations of what each ñana will be like and just focusing on getting into a solid practice routine; incorporate insight practice into your daily life, just like you've been doing with the music stuff, and you'll find that there are many, many deep insights to be found in the most seemingly mundane of experiences.
NA made a few comment which, in my experience at least, aren't quite accurate so I'd like to try to clarify a bit.
NA:
Music requires very narrow focus, which is counterproductive past the second vipassana jhana, so be wary of that.
I disagree with that, applying vipassana while playing or listening to music can be incredibly informative and can be used to go way beyond second jhana. If you're trying to control the attentional focus then you're not doing vipassana properly, there's a natural focus involved in each vipassana jhana and it'll shift regardless of the object being observed.
If you're totally concentrated on playing a piece of music though, then I would agree that a narrow focus is natural, however if you play around with that a bit, you can let go of the effort required to maintain the focus and 2nd jhana can begin to reveal itself. I do this quite a lot when I'm recording guitar or vocal parts so it's not just a theoretical suggestion, it can be a very efficient way to remove even the most subtle performance anxiety and improve your playing by making you much more relaxed.
NA:
I would put on headphones and start listening, but my attention would automatically widen to include the body sensations, the visual field, etc, and eventually music itself would become a distraction.
That's concentration, not insight practice. I recommend trying this again, but instead of taking "music" itself as object, look closer at the sensate experience of hearing and the way in which a holistic perceptual field is created from all the various sensations; break these down, look at
how the mind overlays concepts of timbre, rhythm, pitch, etc, or how the mental images of each instrument happen
after the sound is heard.
In my experience, there's a shitload of insight to be found through using music as either an object of concentration or as something to break down via vipassana.