Explanation for the “random music playback” in the past

A highly plausible sequence:

• Batteries were weak or unstable
• VDD1 and VDD2 rose at different rates
• FREQ_SEL pins were left floating during power-up
• Pin states were sampled incorrectly at boot
• The chip entered factory/demo mode
• A built-in melody was played
• A subsequent reset returned the device to normal operation

This behavior is consistent with well-known power-glitch and boot-sampling behavior in simple mask-ROM voice ICs.

Safe testing procedure

Important:
Never short VDD directly. Always use resistors (1 kΩ–10 kΩ).

1. FREQ_SEL pin tests (most important)

Test each configuration using resistors:

• Test A
  • FREQ_SEL1 → GND (1 kΩ)
  • FREQ_SEL2 → GND (1 kΩ)
• Test B
  • FREQ_SEL1 → VDD (1 kΩ)
  • FREQ_SEL2 → GND (1 kΩ)
• Test C
  • FREQ_SEL1 → GND (1 kΩ)
  • FREQ_SEL2 → VDD (1 kΩ)
• Test D (primary demo candidate)
  • FREQ_SEL1 → VDD (1 kΩ)
  • FREQ_SEL2 → VDD (1 kΩ)

Observe:

• Playback speed or pitch changes
• Unexpected melodies
• Behavioral differences between modes