yeah... more people see that the same way!! (http://www.duolingo.com/#/comment/242768?from_skill=67ae009bfa4cd49017ad877ead41a20f)
Since the IPA (International Phonetic Alphabet) for Brazilian Portuguese is quite self-explanatory, I advise to check you this site http://www.portaldalinguaportuguesa.org/main.html. Type a word on the search bar, then click on it in the results page and you'll find a "Phonetics" section on your lower right.
For those who might be thinking about taking trips to Brazil (or any other Portuguese-speaking country), this site will also give you the pronunciations used in different locations (between Rio de Janeiro and São Paulo, for example).
That's a really useful site. Here's a direct link to the "coronel" page: http://tinyurl.com/kvdnpcz
If I'm reading that page correctly, speakers from some places do nasalise the 'o' (which is probably the effect Andrew48 mentioned), but I don't hear it in Duo's voice or Google Translate's voice.
I doubt you're the only one, but you are one of very few, given that "kernel" is the standard pronunciation, and the only one found in the dictionary. The "nel" probably sounds nasally because it begins with a nasal sound, which bleeds over into the vowel for some speakers. Not all speakers pronounce it nasally, but it's not uncommon for nasal sounds to affect adjacent vowels.