You can use Audacity (http://audacity.sourceforge.net/) to record sound that fits with Arduino.

1. Get the newest version of Audacity
2. On the left bottom corner of the interface, there’s a dropdown menu “Project Rate”, choose 8000.
   
3. Click the red record button to start recording your sound.
   
4. When it’s done, click the yellow stop button.
5. Click the track title drop down menu, select “Set Sample Format” to 16-bit PCM.
   
6. In the file menu, select “Export…”.
7. Select “Other uncompressed Sound” in “save as type” dropdown list.
8. Click “Options” button
9. Select “WAV(Microsoft)” as header, “Unsigned 8 bit PCM” as Encoding.
   
10. Name the file (file name should be very short), and press the save button to save it.
11. Don’t put anything in the Edit Metadata window.

If you want to convert an existing sound track:

1. Open the sound with Audacity
2. If it’s Stereo, click the track title dropdown, and choose “Split stereo to mono”
   
3. Reduce the gain to -6dB for each channel.
   
4. Click the “tracks” menu, and choose “Mix and Render”, you’ll get one channel left.
5. Select the whole track/part you like, copy it
6. Click Files->New
7. Change the project rate to 8000
8. Click Tracks->Add new->Audio Track
9. Paste the track here
10. Follow steps from 5 to the end of “record sound” instruction.