When ん is followed by an あ-line character within a word, the N-sound tends to be flattened out a bit to an I-sound, or even a Y-sound. Mostly this is down to mouth flaps – it’s much easier to say vowel-Y-vowel than vowel-N-vowel. So 千円 sounds a bit like “sei-en”, 原因 (げんいん) sounds a bit like “gei-in” or even “gei-yin”, and so forth. It’s not quite an I-sound, precisely – you still move your tongue up to make the N-sound, but you just don’t quite touch the roof of your mouth with it. People are still going to understand you if you say “sen-en”, but if you want to sound a bit more natural, it’s worth getting the hang of “sei-en” instead.
“Sanni” is definitely a typo, though.