I moved this from the French forum given you wrote in English.
Please look at the pink panel at the top of the page. What version of Audacity? What version of OS X? If you have Yosemite, Audacity 2.0.6 does not officially support it.
Are you importing the same file in 1 and 2?
If after doing 2, you do 1 again, is there still no sound?