Me again. It can come from "skin" problems. Flac files can contain links to pictures, which some players absolutely want to display, or from DRM (Digital Rights Management) associated with flac files. If you bought some music a while ago, you might have restrictions upon transferring these flac files to another device. There is a procedure, which depends upon where you bought them from (and upon whether you still have your login details for that web site, upon whether the web site still exists, etc.).
The simplest way is to convert them all to high quality mp3. It reduces the needed storage space by about half. You can use Audacity, but this is time consuming because you have to open all flac files one by one and then to export them as mp3. The advantage is that Audacity functions on all platforms (Windows, Mac and Linux).
In Ubuntu, the fastest way is the command line.
First, go to the folder and "clean" the file names. That is, get rid of spaces, brackets, single quotes etc. Single quotes really really have to go. A file called "01 Don't eat (my cake).flac" has to become something like "01_Dont_eat_my_cake.flac".
You can use the "rename" command:
rename 's/ToMatch/ToReplaceWith/' *
For example, to replace spaces in file names by underscore :
rename "s/ /_/g" *
To remove brackets you have to escape them:
rename 's/\(//' *
rename 's/\)//' *
Use ls to check the file names. Any file name surrounded by single quotes contains some improper character.
Convert :
For a single file:
ffmpeg -i sample1.flac -q:a 0 -map a output1.mp3
For all flac files in the folder:
for i in *.flac; do ffmpeg -i "$i" -q:a 0 -map a "${i%.*}.mp3"; done
The -q:a 0 means you want the best mp3 quality. The resulting mp3 files are nearly as good as the original files (I can't hear the difference).
Remove all possibly offending flac files from your android device (or test them one by one if you feel like it). Replace them with mp3 files.
