i am working with Flash MX and I often need sound effect and background sound. Does anybody know good application, so I can try it?

Check this site out. It has some programs to edit audio and to convert it into different formats.
http://www.audioutilities.com/

But if you are looking for a program as far as background music, I would recommend
that you check out ACID Pro music creation software. Real easy to use.
They should have a free program still available called Acid EXpress. You can find
out more information by going here. http://mediasoftware.sonypictures.com/products/acidfamily.asp

I use Voyetra Record Producer, as well as Magix Audio Cleaning Lab 2005. Both are very easy, and produce good audio results. Of course, this is if you are a sound enthusiast and do audio restoration as a hobby. My next goal is to record a DVD-Audio disc, with the new 192 Khz sample rate, but I have to wait until I get a Sound Blaster Audigy ZS Platinum sound card.