Important news! I was playing around with android and discovered that you can manually change the default alarm tone without any obscure program like ES File Manager. Just do the following (you'll need adb -- Android Debug Bridge -- or something similar):
1. Copy "/data/data/com.android.providers.settings/databases/settings.db" to your computer. If you can't do that directly (with adb, for example) copy the file to /mnt/sdcard, and then to your computer. Do the same for "/data/data/com.android.providers.media/databases/internal.db" and "/data/data/com.android.providers.media/databases/external-<something>.db".
2. If you don't already have a program like SQLite Browser on your computer, download it from here:
SQLite Database Browser.
3. Open internal.db with SQLite Browser, go to the "Browse Data" tab, navigate to table "audio_meta", and scroll down until you see the file you'd like to have as the default tone. Take note of its "id" field value.
(screenshot)
4. If you don't want any of those files, open external-<something>.db with SQLite Browser, go to the "Browse Data" tab, navigate to table "audio_meta", and scroll down until you see the file you'd like to have as the default tone. Take note of its "id" field value.
5. Now pay attention, it's very important that you remember not only the id but also which file you used: internal.db or external-<something>.db!
6. Open settings.db with SQLite Browser, go to the "Browse Data" tab, navigate to table "system", and scroll down until you see an entry whose name is "alarm_alert". Double-click the "value" field of that entry.
(screenshot)
7. You will probably be shown a string like "content://media/internal/audio/media/39". You may have to change two things: first, the word "internal", and second, the number "39". (Of course you may see a different number in your file)
(screenshot)
8. If you got your id from internal.db, set the word to "internal". If you got your id from external-<something>.db, set the word to "external".
9. Replace the number with the id you got earlier.
(screenshot)
10. Save settings.db and close it. Now copy it back to "/data/data/com.android.providers.settings/databases/settings.db" (once again, you may need to copy first to /mnt/sdcard and from there to where you need). Please notice that this will overwrite the old file. (If you don't want to overwrite it you may consider moving the old file to "/data/data/com.android.providers.settings/databases/settings.db.old")
11. Restart your phone.
12. Try to create a new alarm and see if the default tone is now what you wanted!