Maybe it'd because I am rather deaf, but it sounds to me more like "Ydych chi'n gallu nofio gyda fi?"
I have just listened to the audio above and I agree that now it sounds perfectly OK. I don't understand why it sounded so different when I did the exercise.
Yes, it's probably that -- though I have been learning on and off for donkey's years! Maybe my heading aids are better adjusted today! I think I had better stop commenting on pronunciation!
Could this also be interpreted as asking permission? (e.g. "I like her. Can she swim with me, please?")
gallu/medru - being able to, having the skill/knowledge to
cael - being allowed to (and several other meanings and uses)
- Gaiff hi ddod i nofio gyda ni? - May she come swimming with us? (gaiff hi is from the future tense of cael)