A good quality cdj is designed for just that. Personally i would reccomend pioneer cdj 2000 nexus but they are a bit spendy, and considered a major compedator for top of the line however you can probably find much cheaper ones by numark or dennon or others…older ones will have fewer features but most are designed to at leat be able to quicky scrub through the track and set a cue point to start from and it will save that point untill you set a new one ad have some pitch adjustment. The pioneers of course have way more frills than that which you might not need for your purpose but you might find a creative use for eventually…

The best software for this is the Amazing Slower Downer. I use the iPad/iPhone version, and it’s great. ASD can import a track from a CD (or from iTunes or even Spotify). You can then define and save sections of track as loops. Because the software lets you define the loops by tapping a key to begin the loop, and another to end it, you can create loops that begin and end exactly on the downbeat. When you replay them, they repeat over and over without missing a beat. Learning music from a good loop like this is like having a live musician repeat the same phrase over and over again.

ASD also lets you slow down the music, without changing the pitch. It also lets you change the pitch without changing the speed, so you can transpose it up or down.

The PC (or Mac) version is about $50. I use the iPad version, which is only $15. The only disadvantage to the iPad version is that you can’t directly read in a CD. However, most of the music I want is available on Spotify.

If you want to do the same thing for free, get Audacity for your PC. It\'s not nearly as easy to use, but it\'s free.

I’m not sure if this is your answer because you’d have to rip the song from the CD into a file but Audacity is an easy, free, open source sound mixing software that would give you razor edge ability to isolate portions of a song. 

I’m an amateur guitarist, and here are my suggestions…

1.  Transcribe.  – Great package allows you to loop, show frequency analysis (if you don’t have the score), and even apply custom EQ to highlight or hide parts of the music.  So you could make the cello frequencies more pronounced. 

2. StudioOne Prime from Presonus.  A free Digital Audio Workstation (DAW) that allows many of the same features as Transcribe, but if you have an audio interface to record your cello, you can record it as another track.  This can be great, to evaluate your playing later.  Recording your practice and then listen back can really help you improve when you hear mistakes that you didn’t hear playing live.

3. A small digital recorder like TASCAM DP-008EX.  This is in the same ballpark as a DAW, but does not require the computer (aside from ripping the track in the first place).  I have an old BOSS BR600, that I like using (though I’d like something newer).  The benefit of not being on a computer is that you can keep it away from the computer, and focus on music, with fewer distractions.  

4. TASCAM CD-GT2 is another option, and is actually a CD player.  TASCAM GB-10 is a smaller trainer.

So, lots of options, but I think you might be happiest looking at the options created specifically for musicians.