A note about uploading mp3 sampels.
I realised, that not all mp3 files are automatically labeled with the speaker button. In the ccmixter forum i found an explanation which i post here. It has to do with the bit rate of the uploaded sound file:

“SPECIAL NOTE: Flash (tm)’s MP3 player only knows how to play sample rates divisible by 11 so all the tracks encoded at 48k (e.g. most of fourstones) will not play and so there will be no speaker button. If you have a 48k upload you should use ‘Manage Files’ to upload a 44k version of the MP3 and our system will find that one and play it on request.”
More on that here: http://ccmixter.org/thread/889

Uploading wav files.
If you do that, think about also uploading an alternative mp3 file (44K bit rate). If you do so, one can immediately listen to the file by clicking the speaker icon in the webbrowser. In the downloading section you can than decide if you want to download mp3 or the wav file.