Here is a simple way to rip mp3 from flac files. There are few softwares that need to be installed for to work.
Download and Install Sound Converter For Linux. It can be downloaded from here.
Once Installed you can start up the application from the multimedia panel in the start menu.
Install additional packages
#sudo apt-get install flac mp3splt
Step 1: Convert the flac file to mp3 using Sound Converter. This process is quite simple. Start the sound converter and specify the flac file you wish to convert and press "Convert". This converts the flac file to one single mp3 file. But we are aware of multiple songs in that mp3 file.
Step 2: Split the single mp3 files into multiple mp3 tracks. This can be done using the mp3splt utility we just installed.
#mp3splt -f -c Album.cue Album.mp3
Assuming that Album.mp3 is the file produced by the Sound Converter. "cue" files are generally provided with flac files for this very purpose. If you do not have the cue file, you have to use some mp3 splitter to achieve the same.
Step 3: The new mp3 files might have issues with reading mp3 duration of these files. If that happens then a simple tool called vbrfix will fix the problem.
#sudo apt-get install vbrfix
To fix a mp3 file using vbrfix
#vbrfix myfile.mp3 myfile_fix.mp3
The myfile_fix.mp3 would have the duration problem fixed.