As you see, sometimes, iTunes is not so convenient for transferring music. However, you can try an alternative to iTunes - Leawo iTransfer. It's easy to use and without sync with iTunes. Check below steps.
Step 1. Run Leawo iTransfer and connect iPhone 6s to computer with USB cable.
Step 2. Add music you like from computer to iPhone 6s.
Note:There are 5 ways to add files from computer:
A.Drag and drop the computer files directly into the window of the software;
B.Right-click music library and choose "Add File" or "Add Folder";
C.Click "File" on the upper right corner, and choose "Add > Add File/Add Folder";
D.Right-click music library and click "Add File";
E.Click "Add" button on the top left, then choose "Add File" or "Add Folder".
Step 3. After adding music to the software, it will automatically begin to transfer music from computer to iPhone without iTunes.
Of course, as a dual-way iPhone transfer tool, excepting for transferring music from computer to iPhone, Leawo iTransfer also allows users to transfer music from iPhone to computer for backup.
For more details, check: How to Transfer Music from Computer to iPhone