Import MP3 to play alongside score

You can actually achieve this result using 2.3 + MuseScore Drumline.

You are not importing the audio file, but you able to play it back synchronized with the score.

Here's how to do it.

STEP 1 - Install MuseScore Drumline

https://musescore.org/en/mdl

STEP 2 - Add Sampler Instrument

Create a new score and add the instrument MDL Sampler (or add to existing score).

STEP 3 - Set Up A Rig

Using one of a number of different virtual rigs (Mainstage 3, Ableton LIve, etc.) import your audio file to create a sample and assign to a specific MIDI pitch.

STEP 4 - Configure MIDI Out in MuseScore

Go to Preferences > I/O

From the MIDI Output dropdown select the output that corresponds your rig connection (could be direct or through intermediary, depending on your particular MIDI setup)

Make sure the PortAudio checkbox is checked.

STEP 5 - Configure the MDL Sampler

a) Select the MDL Sampler staff in note entry mode.
b) Select Edit Drumset
c) Create a note that corresponds to the MIDI pitch assigned to the sample in your the rig

STEP 6 - Using the Samples

In the MDL Sampler staff, simply enter the note that corresponds to the desired sample at the appropriate count in the score.

TIPS:

You can break the samples up into segments that correspond to different rehearsal numbers or key phrases. Simply create additional samples on distinct MIDI pitches in the rig and create additional notes in the drumset palette that correspond to each MIDI pitch.

Only now I'm seeing this request, after a little over a year, but anyway, the solution I see posted here, dealing with MuseScore Drumline seems so complex and with so many complicated steps! I've just tried a much simpler solution, which I know works fine, because it worked fine for me.
The problem is: how to play an MP3 file alongside with the score that is loaded in MuseScore?
My solution is:
step 1 - Create an SF2 soundbank containing just ONE sample, ONE instrument and ONE preset. This can be done easily using for example the Viena (with ONE "n", not the Creative Vienna, which requires a Creative sound board to be present in the computer). Make your MP3 file be the only sample of that bank.
step 2 - Open MuseScore and load the score you want to play with.
step 3 - load the SF2 bank you created into MuseScore, using the syntheziser panel.
step 4 - Add a new staff to the score and assign your instrument to it (use the mixer - F10)
step 5 - In the staff, add the note corresponding to the preset that was created (normally it would be C3), in the measure, or note where you want the MP3 to start playing. Make the length of that note match the length of your MP3 file, so that the sound will be played entirely. That can be one or more measures, of course.

I've done this - it's quick and easy and it works fine - it did to me.
Now, one piece of caution: the MP3 sound (assuming it's a piece of music) will have a tempo value that is already fixed, so it should be inserted into a score that has the same tempo figure, or else there will not be the necessary sincronism between the MP3 sound and the rest of the score. Of course if you need to obey the score tempo, you can reduce or increase the MP3 length to adapt it to the score tempo, before creating the sound bank. Any reasonable wave editor allows you to do that.

Didn't we discuss "provide audio backing, just as musescore.com does with video" under other threads, one of which I started, and there was dispute over whether it was useful, what the use case was, or whether I could do it by running another app in parallel? Why are we discussing how to do this with drumline and soundfonts and extremely long notes instead of the obvious, what was asked for by the OP (and myself) in the first place (although the OP's use case is much more compelling, i.e., facilitating incremental transcription).

If these explanations of how to fake it with soundfonts, drumlines, and very long notes are worthy of consideration, than the goal itself is implicitly worthy of consideration, and the easiest way to walk from here to there is to put one foot in front of the other, i.e., "import mp3 to play alongside score" as per the title of this thread.

Although I would use this feature often, it would have to be done in such a way that precluded the output going to "Upload Audio", or the ground rule of the site vis-a-vis royalties would be violated hourly and the copyright enforcers would shut down the site.