As someone who can more or less competently handle programming and design and graphics and can work sound effects generators well enough to get what I want, the one thing that keeps my games from being complete solo projects is music. For a long time, I was just leaving music out of my games. Starting this year, I've released three projects with CC-0 or CC-BY-3.0 licenses for music*. It's elevated the feeling of polish on my projects a lot, I think, and contributes to a more complete experience, allowing me to make games that connect on additional levels.
As much as I'd love to work with musicians to compose for my projects, I don't like reaching out to people to ask them to do music for my projects for free since I value the work they do. But since my games are hobbyist passion projects and literally have no budget and are all released as free (and especially since splitting ad revenue or tips can get messy, no matter how meager it can be inevitably is), I really can't afford to pay anyone. I try to feature the names of the artists I use prominently.
I get a little nervous that the songs I use will end up being used in a lot of projects and if a player recognizes a track from another game may find that disorienting or tacky, but so many people make so much neat stuff that I find carefully selecting OGA music can really contribute to the identity and mood of my games.
I'm really grateful to the artists who make this stuff available to use freely, and I've been thinking that I'd like to develop some graphical asset packs so I can give something back to the community.
*These games are Temple of the Wumpus, Liz & Laz Episode 1: The Control Cubes, and Koi Puncher MMXVIII.
EDIT: Sorry for the old thread bump! Didn't realize how old it was. But it's a topic of perennial interest!