My dd did this as well, and it actually got her mislabeled as PDD at first instead of epileptic with an auditory processing disorder. At three and a half, we've just in the past month got her to a point where she can have conversations (thank you tegretol!!). She's been able to technically speak for awhile, I'd say maybe 2 .5? But I guess it was just too hard to get the words out. She still has some processing issues, and will often say words that ryhme, or wrong words. However, the girl LOVES to sing. OMG, before she goes to bed every.single.night. the whole family has to come lay in the bed with her and we all sing skinamarinky-dinky-dink at the top of our lungs. As soon as it's over, she's out like a light.
Yes, she's a character.
Anyway, around 20 months or so she would start singing, singing, singing, although we'd have no idea what she was singing. We later figured out it was bits of television shows - hence the PDD suspicions.
I wouldn't downplay the effect that epilepsy can have on speech and language usage. Like I said, with dd there were all kinds of suspicions about what was going on, when in truth, it was just that her seizures weren't well controlled - add that to a cognitive roadblocker like keppra and what was happening was that she *really* wanted to speak, I think, but just couldn't because of the mind fog she was in.