Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Launch Text and Alert Tones Store –
Apple just will not stop they have just launched a new section to iTunes which will allow you to buy Ringtones and Alert tones. Customers could already buy Ringtones from iTunes but the Alerts tones is what most people were wishing for
Ringtones are listed for $1.29 and alert tones for .$99. You can access this new store by going to Settings > Sounds > Buy More Tones.