You can easily create iPhone ringtones from Hark audio files using both Mac and Windows based PCs, if you're using Linux see the snippet at the bottom.
Download the sound clip MP3 that you want as a ringtone and add it to iTunes. Once added, right click (control + click) or from the Advanced menu item on the menu bar select the "Create AAC version" option.
Once the file has been created, again right click (control + click) and select the "Show in Finder" option; on Windows you should click "Show in Windows Explorer". Now rename the file and change the file extension from ".m4a" to ".m4r". Remove the MP3 and AAC version from iTunes, and drag the newly renamed file into iTunes. The ringtone should now show up under the Ringtones section of the Library in the left hand sidebar.
Now that you have your ringtone, go to the Ringtones section of the Library in the left hand sidebar and drag the newly created ringtone to the iPhone you want to use it on, just as you would an MP3. You should now see your ringtone on your iPhone, under the Settings / Sounds / Ringtone menu. Select your Ringtone to preview it, and you are all done!
If you've on Linux you can create the ringtone using MPlayer, FAAC and the commands below, past that you are on your own.
mplayer -vo null -vc null -ao pcm:fast:file=file.wav file.mp3
faac -b 128 -c 44100 -w file.wav
mv file.m4a file.m4r
scp file.m4a root@iphone:/Library/Ringtones/file.m4r