iTunes is quite a closed environment where it's supposed to be the tool you use to manage your iPhone's media content. Although Apple is quietly hoping you purchase all your media content from within iTunes, you are able to drag your own content to iTunes and import it automatically.
Of course, that only works when the media content is supported by iTunes. If you've ripped your entire CD collection in the FLAC format, supported by many audiophiles, frankly you are stuffed. Short of converting every single track before importing into iTunes, if you try and drag a FLAC file into iTunes it will do nothing as it's simply not supported. And as it's not supported, you can't upload the file to your iPhone.
A simple answer is a tool that automates this process and gets your data onto your phone with the minimum of hassle. WALTR is the solution.
WALTR isn't an iTunes replacement. It's not designed to manage your content. You can't use it to remove files from your iOS device (you can do this from the device itself). It's simply designed as a powerful facilitator. It will take an audio or media file, in just about any format, will then process, convert and upload it to your iOS device. Audio formats supported include MP3, FLAC, APE, ALAC, AAC, AIFF, WAV, WMA, OGG, OGA, WV, TTA, DFF, whilst video formats are MKV, AVI, MP4, MOV, MPEG, m2ts, 3GP, WMV, H264, H265, amongst others. 4K video is also supported, so if you have a 4K movie, you can get this (converted first) onto to your device.
WALTR also includes audiobook support. Simply drag a M4B file onto the application and it will process and make it available as a book, within the iBook app.
It's worth adding that WALTR includes a lifetime license, so once purchased you don't need to re-subscribe every year.