With midi files, each track can be played "solo".
For other files, reducing any frequencies above the bass as much as possible is the best you'll be able to do in attempting to isolate the bass frequencies. I have found that e.q.'s with 12 or 15 db cut usually do not cut enough.
So last week I was investigating your question based on software found at: Free software Winamp
Some of the links were broken in that thread so I did a little interwebz browsing and found current links for Winamp(a media player) and its user designed plug-ins. The capability of the plug-ins was surprising for such small files.
I downloaded the following: Note: what follows works with Win XP(& older Windows systems).
Winamp 2.xx available at http://www.oldapps.com/winamp.php
(v2.95 lite,full, standard) should work.
Don't use Beta versions. I tested v2.91 Full. Based on what I read, I used the older version of Winamp to be more compatible with the plug-ins that I found. The plug-ins may be compatible with newer versions of Winamp but I did not test them in that environment.
After learning that Winamp uses plug-ins, I then went on the hunt for an e.q.(with more than 15db cut), a looper/pitch shifter and a slow downer. Winamp has a plug-in database that goes back 10 years or more. Plug-ins downloaded: Looper/pitch shifter (point to point w/micro adjustments for the point)
Note: Winamp can loop a whole song w/o a plug-in. Slow Downer(tempo/pitch) E.Q. (100db cut) + Parametric (user selected boost)
Note: Cuts enough to isolate bass ~90-95%. Easier presets to use. This e.q. is the typical draw bar type. This program displays the e.q., you set the draw bars, click "preview" until you get it set then close the e.q. Pre-set files are easy to create. E.Q. (30db boost/cut)
Note: Cuts enough to isolate bass ~90-95%. 128 band. This e.q. uses a graph line that is manipulated with the cursor. The program is displayed and stays on screen. Very small adjustments make big changes in sound. Other Plug-ins
I also downloaded other plug-ins to work with CD's: CD reader CD to MP3 converter Operation Notes
Install Winamp first because the plug-ins use a "plug-in" directory within the Winamp directory. When the plug-in is being installed, be sure to check the display of the directory that it being installed in to make sure it goes to the Winamp\plugins directory. Change it if it displays something else.
You can't use the "slow down" plug-in with the e.q. plug-in. And only 1 e.q. is available at a time. I downloaded a "DSP" stacker plug-in that was supposed to allow you to use multiple DSP plug-ins but loading the e.q. and the slow downer would still not work. The Winamp e.q. works okay for removing mid/treble when using the slow downer.
After Winamp is installed, Ctrl-P brings up the preference screen where each input, output, effects, etc are selected/changed. Your selections act as "switches". Ctr-D increases the size of the Winamp screen display.
For the slow down plug-in, decreasing the speed and increasing the pitch an octave works pretty well to increase the slowed down bass definition.
The plug-ins are very small files and create small, independent screen displays so you'll have several small screens displayed on your monitor all the time.
I have worked with this stuff for a couple of hours and everything worked fine. I was able to slow down a CD in the CD player or an MP3 on the disk. I was even able to slow down streaming audio. I was able to use the e.q's to isolate the bass from and MP3 file and a CD. It seems that you can also output a "slowed down" file but I havent' tested it yet.
The e.q's allowed such a wide range of cut/boost that it took a bit of testing to get the bass isolation/cut just right.
One other great side benefit from all of this was that I found I was able to use the e.q.'s to cut the bass enough to make any song a backing track.
I've barely touched on Winamp's capability but the tools are there (for free) if you put in a bit of time to get it all set up, tested and get used to the Winamp interface.
Maybe there are some Winamp gurus on TB that can add more to this discussion.