First, if you don't already own it, download Audacity. Also have the MP3 you want to convert (duh).
With everything downloaded, find an MP3 that needs to be adjusted. It may be too loud, it may be too quiet, we are here to fix that.
In the top left corner of Audacity, there will be a little thing that says "file". Click on it to reveal a drop down menu. Select "open". From there, go to where you downloaded your MP3, click on it, then hit open.
Audacity should now show blue sound waves. This is the MP3. The bigger the spikes, the louder the song.
In the same top area where you clicked "File", find and click on "Effect". It is to the right of "File".
Another drop down menu will appear. Don't be afraid of all the options, and simply click "amplify". It is towards the top.
Something should pop up, refer to the third image I provided. First, make sure the box is unchecked, if you have it checked then your song could become distorted.
I number will be displayed. If a positive number is displayed, your song is too quiet. If your song has a negative number, it is too loud.
Click "OK". Audacity will now begin applying the amplification.
Once the amplification process is finished, click on "file" again. On the drop down menu, select "Export Audio". DO NOT CHOOSE SAVE PROJECT. Choose somewhere to download the MP3.
Close Audacity. It will ask if you want to save project, select "No".
If your MP3 is STILL too quiet, repeat steps 1-3. Now, on step 4, check the box that says "allow clipping". From there, little by little (I would advise no more than 1 at a time) amplify your MP3 and hit "OK". The reason for this is your song, at it's loudest point, will become a tad bit distorted. If you REALLY want your song to be louder, know it will be a bit distorted. Play your MP3 on Audacity to make sure it is the correct volume.
*ONLY PREFORM AFTER AMPLIFYING ONCE*
If your song is STILL too loud, complete steps 1-3 again. On step 4, it will recommend you amplify it by 0, as you have already amplified it. Instead, amplify it by a negative number (no more than -1 at a time. Then, play your MP3 on Audacity to make sure it is the correct volume.
I have this exact same problem, but with voice packs as well as music. As far as I can see, you want to make your sounds and music very loud, because Smash has some sort of "quieting" effect for the sounds and music.