How do you "see music" or "hear colors"?!
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I can describe this experience being sober. When you see something so pretty and colorful or hear music that you love, then you can hear it, see it, feel it, smell it, because it is so overwhelmingly beautiful that it takes over every sense.
It's basically like having a hallucination where you can experience things that you can't normally. Sometimes when music is played, people see colours or patterns but it is only a hallucination and "hearing colours" is just hearing music that, whilst experiencing a "trip", they associate with the colour.
Hope I helped :)
Synethesia. This is a mental disorder that gives you the same feelings.
Basically seeing purple automatically makes you hear a certain music note, and other similar experiences.
http://en.wikipedia.org/wiki/Synesthesia explains it all.