Record the background using the styles of your choice, and play it back as an MP3 while you add the vocal, trombone, whatever line to it.
It's a novelty - so the crowd will eat it up no matter if it IS the same background everytime you play it. Vary the lead part - that'll keep it fresh.
Personally, I LOVE getting out in front of the KB once in a while to do some "front work". Now that my sax chops are coming back ( that's RIGHT, Boo !!!! )
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I'll be doing LOTS of cool instrumentals...like: Harlem Nocturne, La Vie En Rose, Yakkaty Sax, and any Earl Bostic arrangement I can figure out !
I'm excited !