Some games support custom soundtracks:
Music files have to be .MP3. They can be saved anywhere on the SD Card. You can't do nice things like have certain folders for certain games, it just finds all the music files. You select one song at a time, or tell it to pick one at random.
You can also hack-in your own music to other games:
When I got my Wii I spent a bit of time agonising over rechargeable battery packs. There are a bunch available, but not much concrete info on them. Took me a while to realise that the clear best option is to just by good quality rechargeable AAs, namely Eneloops. You can get little starter kits with a charger and a quantity of AAA and AA cells 👌
Highlights / curiosities from that list:
Wiis are very easy to mod, which lets you do stuff like reenable online services and play, and run games from external drives and the SD card.
RiiConnect is a way to use the online functions of the Wii even though those services are now shut-down :)
It's a simple script you run that generates some files, that you then copy onto your SD card, and install via the Homebrew Channel/
I also had to install the dependencies
xdelta3 to get it to run.
It works for games too! There's a general patch that works for most stuff, but some particular games have their own patch, which does some extra stuff like change text in the game data.
You can easily convert an ISO to a WBFS file with the command-line tool Wit (or using a GUI frontend for it) using the copy command. Simply
wit copy [filename].iso [filename].wbfs will do the job quickly!