I wouldn't trust a lossy format such as MP3 to represent a signal with 100% accuracy. Even if the original signal you encoded was absolute silence, it may no longer be the case after MP3 encoding due to rounding errors, filtering artifacts, etc.
For PCM files it's easy to check if the data stream is filled with zeroes or not. Unless you're 100% sure you're ...
Firstly try to choose HDMI in the Volume control.
If you have a built-in speaker in your Acer Monitor and no sound ... try to use the
raspi-config program in the terminal.
Select the "Advanced Options" and "Audio"
Choose the Audio output: HDMI 1
The quality of the output jack is not the greatest even with an amplifier or headphone that can take the line line level output (it also has an odd software pop and been known to miss the first second or so of sound).
Quality is greatly improved by using a I2S HAT that outputs either line level or headphone level signal for you but it does obviously increase ...