Windows is wrong; DiskUsage is right
Based on your question, and on the comments on the question: It is clear that Windows cannot see all the information stored on your phone, but that DiskUsage can see it all.
You need not do anything yet
I'm trying to free up space for more music and pictures
You still have 1 GB free. If this remains true forever, it's no problem.
I suggest keeping an eye on your free space.
Once your free space falls to about 500 MB, you'll be unable to install new apps. I think you may also be unable to upgrade existing apps.
Once you become unable to upgrade existing apps, it's definitely wise to take action. This is because it's wise to always keep security-sensitive apps (such as Chrome and WebView) up to date.
Buying a MicroSD card
Because your phone has a MicroSD card slot, consider buying and installing a MicroSD card and moving some stuff (music?) from your phone to the card. This will cost you money but may save you time.
Choosing a MicroSD card
Another thread provides advice on how to buy a good (non-counterfeit) MicroSD card for your phone.
Perhaps you can upgrade your phone to an operating system which supports "adoptable storage". Such operating systems include:
- Android 6.0 "Marshmallow",
- or CyanogenMod 13.0 "Marshmallow",
- or LineageOS 13.0 "Marshmallow",
- or better.
Certain other "custom ROMs" also support "adoptable storage".
I do not know whether your phone supports any of these operating systems. I have not checked. To find out, you can do a Google search, or you can post another question.
If you can do such an upgrade, consider doing so before you insert the card for the first time. The operating system will offer to reformat the card for adoptable storage.
Before upgrading your operating system, and especially before switching operating systems, I strongly recommend that you back up all your data. You can back it up to the cloud or elsewhere.