Happy, "pitchy" is a word used ubiquitously on singing competition shows and by a lot of vocal coaches. They use it to mean the singer's vocals were not consistently pitch-perfect, i.e., their pitch tended to waver.    So the adverbial form would be "pitchily."

It may be slang, but it seems to be pretty common in usage these days.   (Actually, the word "pitchy" has been in the dictionary for a long time as an adjective referring to the substance pitch - e.g. meaning resembling, full of, or smeared with pitch. Or it can mean dark, as in pitch black.)

I'm betting there are other adverbs that have been contributed in this thread that would also count as slang or neologisms.