Sound is a vibration that travels through the air, and we don't spell sounds like we do words.
Instead, we use words to *describe* the sound:
* Toot
* Whistle
* Chirp
* Trill
Or we can use words that convey the feeling the sound evokes:
* Sweet
* Melodic
* Haunting
* Cheerful
Ultimately, the sound of a flute is best experienced by actually hearing it!