Found no output devices

Two weeks ago, my MBA didn't make any sound and he didn't find any output devices. It was solved by resetting PRAM. Today I have the same problem, but the reset doesn't seem to work. If I restart, I get a black screen, normal screen , black screen and so on.
When I check by hardware -->audio: it says audiocode:0
Could it be possible to solve it or not?
And if not, could a usb sound card be a solution?
Thanks!

Did the test, found no hardware problem and the sound came back after the test.
A relief!
Thank you!

And here we go again... no sound.
The test said there was no hardware problem, but I have no idea what the problem could be.

Still have no sound. Sometimes the sound is coming back, but this happens more infrequently.
Also, when I try to play any kind of video, there is no sound obviously but the video doesn't play after a few seconds.
Anyone an idea what it could be? Thanks!

Doesn't seem to work. Very weird...

Doesn't seem to work either.
This seems destined to remain a mystery.

I tried another account but the problem persisted.
I'll try the install later tonight.