You need to show us the exact command you are using. "It doesn't work" is not enough information to diagnose problems with something is wildly complex as ffmpeg. Also, if possible, linking to the resulting media file would help too.
The resulting media, if successfully encoded, must have an audio and video bit-rate. The problem very well my be with the tools you are using to read the file.
Update!
It looks to me like you forgot the units on the -b parameter. Try this:
ffmpeg -i 'in.wmv' -s 1280x720 -b 256k -ar 44100 -ab 64k -ac 2 'output.mp4'
Note how the -b switch value has a k on it.
Also, I've just got to say, the size 1280x720 is awfully large for such a low bit-rate. I would highly recommend a size either 1/2 or 1/3 of 1280x720, or increasing the video bit-rate to 512k or 768k. You are not going to get what you think you are going to get with such values.