Tell us about yourself!
No other reports. What does the noise sound like? Are you using an UNO? None of the official Arduino boards have speakers on them.
Best to find someone local who can build them for you.
Contact me at firstname.lastname@example.org.
Resistor to ground gives more consistent mixing when used with different amplifiers.
One end of the resistor network is tied to ground to form a reference point for the passive mixer. This is necessary for the mixer to work with various amplifier inputs.