Volume/Level Boosting of .mp3 files


I have the vast majority of my music as MP3s that were ripped years ago from the original CDs.  They are stored in iTunes, and I've taken considerable time updating album art, ensuring proper titles, etc.  I generally listen via either Bluetooth or direct connection in my car, or with stock apple earbuds (I didn't like the Airpods).

When I compare the MP3s to music purchased on iTunes, the volume in most cases is significantly lower.  I know there is volume increasing software out there, so I wanted to put it to this group (or take suggestions on where I can pose a similar question) for recommended paths.  

Thanks in advance. 
lrschachter
If it’s a level issue, you can use Audacity (or other apps) to uniformly increase the level, just be careful not to exceed 0db, or even keep it at -1db. To preserve the original relative levels between songs on a single CD do all files at once or as one single file if possible.