it is generally not “loudness” that will help with Siri or any other voice assistant. It is tone and annunciation that helps them understand. The louder you get the worse your annunciation and the worse the interpretation and the harder it is for a voice assistant to parse what you are saying.

Speak calmly with not too much tonality or pitch shifting.

Side note: it definitely becomes more difficult when the language you speak to Siri is different from your native language and your annunciation may be off, speaking in an exaggerated accent may help.