Audacity is a great audio editing software. I use it mainly too boost bass and treble which facilitates hearing certain notes. This program combined with Audiostretch are create a solid platform for arranging. I usually open a the song I'm about to arrange in audacity, edit it there, with some effects mentioned below, and export it afterwards (you'll need to install some plugin to export mp3 files however; but it's pretty easy to do, just follow the steps).
Some tricks I use:
Boosting Bass and Treble:
Under the "effects" menu you can find "bass and treble". If you click on it a menu will pop up, on which you can choose to boost ether bass or treble. I usually boost one to the max and the other to the very minimum. For example bass to +15db and treble to -15db. And i don't use the level, I don't know what it does honestly.
This is how the window looks like. (my program is in Swedish
don't know if you can change it, but the window should lok the same but in your language)
You can also use the equalizer under the "effects" menu for the same purpose as the boost, mentioned above. I rarely use because I found that the boost does the same job but better. With the EQ you get a more extreme sound I feel like (fart-like bass, really sharp treble), rather than boost which is more smooth. It's very easy to use, just raise or lower the notches, maybe raise the bass a lot and lower the treble if you want to hear the bass better.
This last trick is a quite useful one. Under the "effects" menu you can find "change pitch". Click on it and window will pop up. What this effect does is that you can change the pitch of a section or the whole song. How is this useful, then? I usually use it in combination with the boost. let's say I have boosted the bass up but it's still hard to hear some notes in the bass. What you can do now is to change the pitch to one octave higher, making the low bass notes much easier to hear. You can also do this with the treble as well, just do the the other way, lower all notes by one octave.
As you can see in the picture I'm changing the pitch from F1 to F2 (one octave).