There are some wonderful festival in the Summer time. I remember jazz in Brussels near the Place de la Monnaie. I think Antwerp is more for underground and experimental music though.

I honestly don’t know, but you should go on www.travelpunk.com and post the same question. that site is THE best place for travel advice, the people on the boards are very knowledgable and couldn’t be nicer!

Brussels:
you can visit this page you will found cultural activities taking place in brussels the next week (you can subscribe to have the news letter every week):

http://ns6150.ovh.net/Residents/vasy/Kernel/private/showMailing.php?ID=36&viewing_code=bc8552be3ba2aaa1c47f2c36156878b4_36

Jazz in Brussels:
you can ask for info at the Flagey wich is old “house of radio” (place sainte croix à 1050 Bruxelles near place Flagey) they have lot of activities about music and jazz:
http://www.flagey.be

Liège:
there is “la maison du jazz” where you can found lot’s activities about jazz:
http://www.jazzaliege.be/

http://www.jazzinbelgium.com/

Luxembourg:
at the moment there are lot of cultural activities in Luxembourg while the town is “capital culturel européenne 2007” (lot of things with astonishing good qualities for a little town…)

http://www.luxembourg2007.org/

warning
I’m only talking about what i know this is not an exhaustive list