Distance Readings Getting Manipulated When Calling Unrelated Function
The tone function uses timer interrupts to toggle a GPIO pin to make noise. That's quite intense from a CPU point of view and is most likely skewing the timing of the pulseIn function.

I would suggest not using tone if you can help it. Maybe change the frequency of a PWM output to something suitable and use analowWrite to create a 50% duty square wave.

Also learning how to use the Input Capture peripheral of the MCU (read the datasheet) will enable you to perform asynchronous sampling of your pulses and obtain much more accurate results.