I would have said that the "I don't know" sequence matches this description -- it's just the tonal contour of the original phrase, with the other details stripped out. I don't see how that differs from Punjabi syllables preserving their (originally) allophonic tone while losing their (originally) phonemic aspiration, such that the tone becomes phonemic.
> I wonder if that's really the only item of that type in English.
Obviously I can't guarantee that, but I think it's pretty likely for several reasons:
- I can't think of anything similar.
- The yes/no question answers (including the "I don't know" sequence) are a clear conceptual group and differ from normal English in a few ways. The "no", "uh-uh", is also (I conjecture) unique, being the only English lexical item to contain a phonemic glottal stop. (Plenty of English words feature an obligatory glottal stop, but that stop is allophonic, as when the /t/ of "kitten" is transformed into a glottal stop by the /n/ that follows it.) It is of interest that the only phonemic glottal stop and the only phonemic tone should occur in these two closely related words. I strongly suspect that this group of answer words has these unusual features because of the constraint that you can pronounce them without opening your mouth.
- It's a single word that contains a lot more information than one English word should. For the Latin word "nescio" to mean "I don't know" is unsurprising; all Latin verbs are marked with subject agreement and in the case of a first person subject that is generally the only subject marking. English subject marking is almost always mandatory. This is just more evidence that the tone sequence is quite different from normal words.
- English has a non-vestigial system of grammatical tone at the sentence level; tonal marking is obligatory for yes/no questions. But sentence-level grammatical tone can't really coexist with lexical tone. So I'd be very surprised to see other instances of lexical tone outside very special cases. (For example, the phrase "I don't know" can't ever be a yes/no question, and so can't conflict with the English interrogative tone.)