1
general::
Speak up, as in Finally she piped up, "I think I've got the winning ticket," or Pipe up if you want more pancakes. This term originally referred to a high, piping tone. [Mid-1800s]
American Heritage Idioms
2
general::
Phrase(s): pipe up (with something)
Fig. to interject a comment; to interrupt with a comment. • Nick piped up with an interesting thought. • You can always count on Alice to pipe up.
McGrawhill's American Idioms And Phrasal Verbs