I have jammed with mp3... when I say I only know 2 songs, I really should say I only know two known songs: "House of the rising sun" and "Knocking on Heaven's Door". But I have jammed with 6-7 no name songs, playing along, and those I know too, of course.
With bass, my first line is Pink Floyd's "Money". I mainly learn music theory, scales, etc (which I then apply to guitar!!) I have a nice toy to jam with my bass now (There is an equivalent for guitar):
Tascam MP-BT1. You can get same effects with computer software, but I like it being portable. This allows me to play bass anywhere in my house.
For guitar, my new toy is
Jamlab. I didn't really need it, but I had to pay 20€ transport as I wasn't in the free packaging yet, so I got this, which is 29€.
About musicians, maybe I'm wrong about my area, but there don't seem to be jams, or open mic nights. There are some jams in Barcelona... I could go there. Will have to go some day and ask about musicians entering. The guy who sold me the POD XT with bass extensions was a musician that left Barcelona to Berlin to be able to do something as he had lots of trouble here to find anything. On the other hand, my bass teacher plays in a band that play new-Cabaret and with the classes and that he seems to be doing well.
On the other hand, on my town there are two rock bands (5000 people town) that I know of. But, haven't got them to tell me yet "yes, you can come to jam with us". Here many bands just have time for once a week training, and they prefer to use it for that.