Newbie questions about audio interface

I’m interested in buying some gear just for playing at home/enjoyment (haven’t bought
anything yet).
My plan is to use an audio interface, active speakers , pc , microphone
and instruments (electric guitar + synthesizer).
I’ve never owned an audio interface, so am curious about two points
(lets take the UR44 as an example of an audio interface I could purchase):

(a)I would like to take a song / music (digital format eg on USB stick) and
have this play as background music which I would play along with

  • how does one achieve this ?
    (b)I opted for the audio interface -> active speakers combination to avoid
    buying a eg guitar amp. Without the guitar amp (which has some basic sound effects),
    how does one add effects to what is being played / heard through the speakers ?

If it is just for live performing with audio sources like guitar, synth and a microphone then I wonder whether a small mixer might suit better than an audio interface? On the other hand if you want to record your performance on the PC or use the PC as a sound source (via Midi from your synth) then the interface will be necessary.
Your PC will have no problems playing either wavs, mp3s or even midi file songs (with a suitable program) either from a USB stick or from the hard drive and provided your guitar effects are added in the line before input to the mixer or audio interface you shouldn’t have any problems.

The UR44 comes with a light version of Cubase. You could import the track you want to play along with, and then monitor your instrument input along side it, and there you go. Plus you could record it. There are lots of YouTube videos that cover this basic stuff.