For the musical definition (a melody sung or played above another melody):
* Kontrapunto - This is the most accurate translation as it directly refers to counterpoint, which is the musical technique that descant falls under.
* Pang-itaas na himig - This literally translates to "upper melody," but it might not be as specific as "descant."
For the general meaning of "to elaborate upon" or "to discourse at length":
* Magpaliwanag nang detalyado - This means "to explain in detail."
* Magsalita nang mahaba-haba - This means "to speak at length."
The best translation would depend on the specific context in which you are using the word "descant."