I am jailbroken now on My Ipad 2 with ios 5.01, but I clearly remember missing too many skype calls before I was jailbroken, because I did not hear any rings. Only now, am I looking at system logs with system status. It is weird, as sometimes, I get a ring, other times, I do not. I have the same issue on my jailbroken Iphone 3gs. When I do not get a ring, I get a message notification. I then use the system log feature of the system status app, and see this entry:
SpringBoard  (Warning): No alert sound found at path '/private/var/mobile/Applications/F2CBA25F-9915-46D3-81A7-8D5348A53EE3/Skype.app/call_ringin-30.wav'. AudioServicesCreateSystemSoundIDWithPath() returned: -1500
The file call_ringin-30.wav exists in the Skype.app directory and is correctly permissioned. The ringer works sometimes and fails sometimes with the default wav file. It also works sometimes and fails sometimes when I rename custom ringtones to call_ringin-30.wav
Is this a known issue with Skype?