I agree with what Sdcampbell says, but please let me add on.
You will need to apply several coats of primer and sand lightly between each coat starting (I would say 5 - but you might need more). Between every coat you will want to sand with 300 and working up to 600. Make sure to use a block so you don't get any dips. This is to make cabinet as smooth as possible. Also make sure to dust the speaker down between EVERY coat. Use the primer to fill small scratches. Anything bigger use a wood filler.
Then once the cabinet is PERFECT you can start to paint it. When you are painting it is not the time to be fixing and filling. Once you start painting it black you are going to see EVERYTHING, and it will only get worse once you clear-coat it. You will want to start painting with a FLAT black paint. Make sure that the can is good and shaken to avoid gobbing. Don't try and put to much paint on in a coat. Lots of THIN layers is the key. I would say another 5 layers of paint would be good.
Then you will want to clear coat it. This is were you will get your gloss. The more clear-coat the deeper the paint will look. I should also note, if there is ANYTHING wrong with the cabinet, now you are going to see it. Thus the importance in taking your time.
I will not lie to you and say it is easy to get a really good piano gloss finish. It takes work, but is worth it. Take your time and have fun. Also you will want to be painting in a warm room. If you are using a garage make sure it is warm in there. If you are not up for all this work, you could try taking it to a auto-body shop and get quotes, The can do it, but be prepared to pay, You could also try a Piano restoration service, the should be familiar with recreating the Piano gloss finish, and probably will be cheaper than the body shop/