Which city in the Philippines is the best to retire in - and which city in the Philippines has the best music scene for foreign musicians?

I'm no expert for sure, but I would guess with Manila being the largest, you're going to have better opportunity there. Kinda like Bangkok here in Thailand having more opportunity than, say Krabi.

I liked Cebu. Great scuba diving nearby!

Most people seem to prefer Cebu as it is big enough to have major facilities but small enough to get around. As Cebu and Manila are really the only major international cities suspect they would have the most music options - although music is part of the culture and everywhere in the Catholic provinces.