Connecting MAX9814 to Speaker

I am trying to build a fetal heartbeat sound detection using Adafruit AGC Electret Microphone Amplifier – MAX9814. I attached the MAX9814 mic to the hole of wood fetal hearbeat stetescope (picture attached).

Parallel the VDD and GAIN to get 40db output. The battery I use is two 1.5V button cell connected in series, so I get 3V power to drive the MAX9814 modules. I use the wiring from this [link][2] (picture attached).

Everything is ok when the nurse using a earphone to hear the fetal heart beat.

but when I connected the MAX9814 Modules output to the AUX input of a speaker like this one (picture attached)

I get hissing or buzzing sounds. I know the adafruit tutorial suggested for not to connected the output to any speaker or I’ll get horrible feedback effects. But is there are any suggestion to able connect this MAX9814 output not just only to earphones, but speakers too? so does the couple can hear the fetal heartbeat sounds together with speakers.

I also play with PAM8403 amplifier module like this [one][6] (picture attached)

I connect the MAX9814 mono output to the L input of PAM8403 and the MAX9814 ground output to ground of PAM8403. I soldered small speaker (didnt know how much watts its are, but I salvaged it from the speaker I notice before above) to the L output only. So does I only using 1 channel of the PAM8403 output. connect the 5V power supply from 3.7V 18650 battery (boosted to 5V using DC-DC step up) (picture attached)

for your information, the component between stepup and 18650 battery is TP4506 Lithium battery charging module. and the component with yellow and blue wires is 3.5mm jack female (picture attached)

I connected the L and R of the 3.5mm jack female together with yellow wire and the yellow connected to the L input of the PAM8403 Module. the ground of the 3.5mm jack female is connected with blue wire and the blue wire is connected to the Ground input of the PAM8403 Module.

I test the DIY speaker first using my smartphone playing Tones and I-Dance Monkey song, and it works like charm, but at max turn of PAM8403 potentiometer and max volume of my smartphone, the sounds like distorted.

After several tests, the speaker work as good at lower or middle volume, but when I increase the volume to maximum turn of the potentiometer, the hissing or buzzing sound is back.

Any thoughs whats wrong with it? at lower or middle volume, the sounds too slow to hear the heartbeat (I use my heartbeat for the test, I attached the wooden stethoscope hole to my chest) so I need to play it at max volume without hissing or buzzing sounds. Or there are a trick to reduce the hissing or buzzing sounds?

Please apologize my bad quality of the pictures and my english, as this my first question in this forum.