Re: Overriding ringtone with a custom sound
 

Arto Rusanens eSpeakCaller worked. I don't know what is different. The only problem is that the ringtone doesn't get muted when it says the caller's name, which makes it quite hard to hear it. But I think this is already quite nice.

Re: Overriding ringtone with a custom sound
 

That's easily fixable by editing the ringtone to lower volume between seconds 3 and 6, e.g.