An audio file should satisfy the following requirements to be played correctly:

• Sampling frequency — 8000 Hz;
• Number of channels — 1 (Mono);
• Code size — 8 bit;
• Codec — A-Law.

Audio file might be prepared through different methods:

1. Through "Audacity" audio editing software or its analogue such as "Sony Sound Forge";
2. Through console utilities (sox, ffmpeg, gstreamer);
3. Through online services.

The example of audio file preparation in "Audacity" audio editing software is shown below.

Preparing an audio file in "Audacity"

1. Add a file to the project.
   
2. Split the track into two (transform it into to monotracks) — select "Split Stereo to Mono" in the track management menu.
   
3. Close one of the tracks in the track management menu.
   
4. If necessary, cut the track to the needed length, you may aslo cut out repeated part. To do this, select unnecessary part and click "Delete".
   
5. Change the project's sampling frequency to 8000 Hz at the bottom of the track management menu.
   
6. Change the track's sampling frequency in the "Track" menu → "Change track sampling rate...".
   
7.   
8. Export the audio file: "File" → "Export" → "Export audio".
   
   

   In the displayed window, set:

   • Folder in the file system to storage the audio;
   • File name;
   • WAV title (Microsoft);
   • Codec A-Law.
   
9. Delete tags and finish the export.
   

The file is ready to be uploaded as a ringtone.