What kind of music. I assume from the "sit-down" reference that you are not refering to a rock and roll beer drinking type place. In downtown Raleigh, your main options for live music that do not involve standing up, as well as waiting til midnight for the music to start are Amra's and Yancey's.
Yancey's is a large place with decent food and a great atmosphere. It has great music, especially on weekends....more R&B or "smooth jazz" on weekends than true jazz.
Yancy's - Good Music, Great Food, and Large Times - Fayetteville Street, Raleigh, NC (broken link)
It is on Fayetteville Street in the heart of downtown.
Amra's is in Glenwood South. Depending on the band, it could have jazz or more covers.
Amra’s Raleigh
For true Jazz, there is always great jazz at the bar at Sullivan's steak house in Glenwood South.
You could also check out Tir Na Nog, which has everything from bluegrass to irish. More of a pub atmosphere.
Home - Tir Na Nog -Irish pub
I am sure there are more that I am not thinking of.