10uF electrolytic - signal path
A coupling capacitor sits between two circuits to pass AC signals (audio) while blocking DC voltage.
The ESP32 DAC outputs 0-3.3V centred at ~1.65V DC. Without this cap, that DC offset enters the PAM8403 and causes a loud thump on power-on and distorts the audio. The cap strips the DC, leaving only the audio waveform.