I'm using iPhone 6 and iOS 11.4. I used some online converter sites to convert a youtube URL to MP3 file and a local MP4 file to MP3/M4A file. Then, i added the converted file to iTune library to sync with my iPhone. After I've synced it, everything seemed fine cause the file appeared in the playlist i added it to on both iTune and the device. But, when i checked my iPhone, i couldn't see it in the Song section or the playlist i added it to.
At last, I could copy the file to my iPhone successfully by using a different converter site to convert the Youtube URL to MP3 file. The site is https://www.onlinevideoconverter.com/video-converter. I don't know why. I think the audio quality of the converted file is the culprit. The default audio quality that the site used is 192 kbps. For some other sites that didn't work showed 128 kbps for the output file.
