Rockbox is the only alternative firmware for not only SanDisk, but many other players as well. Many people like it; some don’t. It has many, many customizable settings, more than what the average person will ever use and can be a bit difficult to understand. But there are a number ot “themes” that people have created giving you a choice of many different GUI’s. You can even have several loaded and switch between them at will.
I don’t know of any software that will group by similar beats oer minute, but MP3Gain can “normalize” volume levels, so they all play at a similar volume. Note you have to make these changes to the files while on your computer, then transfer them to your player. One nice thing about this is that the changes are not permanent, You can change the decibel levels or even remove the settings if you want; it doesn’t make changes to the file itself.
I use it on all my mp3 files before transferring to my players. I think the default setting of 89 db is a bit too low though, so I usually use 92-94 db.