I have about 1200 mp3 files (about 45 minutes each) that contain lectures (not music) I used some ffmpeg commands to speed them up by 1.5x and to convert them to Mono instead of Sterio (to reduce file size to fit in a cd).
I used these two commands (in two stages):
ffmpeg -i source.mp3 -filter:a "atempo=1.5,volume=1.2" -q:a 100 first_stage.mp3 ffmpeg -i first_stage.mp3 -ac 1 -b:a 32k final.mp3
After I performed this in batch, when I put them in my car mp3 player, I noticed that all the files show maximum of 7 minutes duration, but they play all the file with no problem. My car mp3 player shows a countdown , and I used to look for the remaining time of each file to know whether I start a new one or not, etc.
Now, all the files start with 5 to 7 minutes (instead of 30-45 minutes), and it counts down to zero and remains 0 to the end of the file, so I complete the lecture blindly with no timer, which is annoying for me.
What can I do to fix this, i.e., to let the car player read the time correctly?