Accompaniment

This probably isn't the best forum to be asking this question in.  Welcome to the forums by the way.
 
I'm not quite sure what you mean by selecting pre-recorded midi style tracks.  MIDI is musical instruction (directions for making a sound), but is not something that has a sound.  These instructions need to be sent to a synthesizer and the synthesizer creates sounds. 
 
The Cakewalk products provide multiple ways of creating MIDI data, using the keyboard, a MIDI keyboard (like a MIDI piano) or by drawing them with a mouse on the screen.  These note instructions can be saved on one or more 'tracks' within a project.  Each track in a project can be sent to a different synthesizer or to the same synth with a different patch to make each track sound like a different instrument.
 
Each cakewalk product also has one or more synthesizers that can be made to create sounds based on the instructions in the MIDI tracks.
 
MIDI tracks can often be found on the internet.  These can be imported into a Cakewalk track within a project.  These imported tracks can then be routed to a synth to make sounds.
 
As to 'entering chord changes' - you can re-type a paper on a typewriter?  you can edit the MIDI file using the keyboard to make individual note changes, redraw the notes using the mouse and keyboard, or just re-record the track by re-performing part or all of the piece.
 
 
 
The quality of the sound that is produced will depend on the quality of the audio components you use on your computer.  Listening to the output from a project on a laptop's built-in speakers will make anything that the synths create sound like crap.  A good quality audio interface feeding a good audio amplifier and a quality set of speakers will result in professional sounding results with any of the Cakewalk products.