"Glywais i mo'r rhaglen."
Translation:I did not hear the programme.
Shouldn't this actually be chlywais i mo'r rhaglen? I thought the negative short forms took an aspirate mutation if possible, and softening otherwise.
You are right, chlywais i ... would be correct. Some people do not use the aspirate in this context in informal speech, but others regard that as being slangy, rather than just colloquial. No reason why Duo should not accept the aspirate, though.
(The reason for the mixed mutation is that is what follows the negative particle ni, the ghost of which hovers in front of negative verbs but which is rarely used in colloquial speech or writing. )