Situation in fiction writing. Two characters are involved in an exploration of a long-abandoned gold mine. However, dangerous creatures may still lurk there. The two characters will be communicating over the course of a few hours but they need to whisper to avoid being detected. What is the best way to handle this regarding dialogue tags? I want the reader to be aware that they are whispering, but I don't what to have to add that tag after each bit of conversation.
"We need to be quiet in here," whispered Jim.
"I agree," replied Bill quietly.
"How long do you think we'll be in this place?" asked Jim, in a hushed tone.
"I have no idea," whispered Bill.
"You're kidding!" said Jim too loudly.
"Shhh!" growled Bill as quietly as he could. "You need to whisper in here, or else!"
How often do I need to remind my readers that the characters are whispering (or should be doing so)?