Thanks. My hearing has gotten bad enough that I miss the subtleties in a lot of these Duolingo recordings.
This is similar to what happens when prepositions like 'ב' and 'ל' precede a definite, articular object (that take the Hebrew article, usually encoded/grammaticalized as 'ה')—except that in this case we're dealing with an interrogative pronoun ending in an open syllable with a fricative and a vowel: איפה. That might factor into the tendency to use less or no aspiration when speaking at a conversational pace. Some Hebrew speakers and dialects already seem to use less aspiration than others anyway, which could be one of the biggest factors. I've had to adjust to that when listening to some colloquial, conversational dialects and speech.
Edited: I made a couple edits for accuracy and clarity.
In a sewing context I think you will still say חוט (I don't sew and therefore don't know the intricacies), but in other contexts there are other words:
- Thread as part of a rope or wire rope: סיב (siv)
- Thread i.e. set of messages in an Internet forum: שרשור (shirshur)
- Thread in computing: תת־תהליך (tat tahalikh), although usually we use the English word
To AdamReisman: according to Morfix thread is also חוט and a spool of thread is סליל חות pronounced slil choot.