add mp3's and at least 1 photo to your sd card, insert it into the wii,
go to the photo channel, click start slideshow, press the 'A' button go to change settings, click change song,
u will now see all your mp3s, you can click one to play it or scroll down and click 'ALL MP3' to play them all starting from top and desecending down the list.
use the + and - buttons to change the volume while the song is playing
this is the only way to do it so far untill nintendo hopefully release a dedicated music channel with all teh options, playlists, shuffle etc......