That’s an interesting concept, but there are a few practical hurdles. The biggest one is how you’ll read the tape. A real cassette deck is still the easiest way to get a clean audio line out; trying to read the magnetic strip directly with a DIY sensor will give you a lot of noise and timing jitter. If you can get a small cassette deck module or a few spare parts, you can tap its line‑out to a microcontroller’s ADC. Next, you’ll need a tiny display – a 0.96‑inch OLED or an LCD that can drive a few hundred images from an SD card. The MCU will run a small loop that watches the audio input, looks for a beat or a cue signal, and then changes the image. A simple threshold detector or a basic FFT could work. If you want a more reliable sync, consider having a few “cue tones” inserted into the tape when you’re recording the slideshow so the MCU can latch onto those exact moments. Power is another consideration. A single 3.7‑V Li‑Po can run the cassette deck, the MCU, and the display for a short play session. A small rechargeable pack with a USB‑C port will make charging painless. Sketch a block diagram with: cassette deck module → audio amplifier → MCU ADC → image buffer on SD → display controller. Add a small speaker for the audio, maybe a simple push‑button for “next” or “previous.” That gives you a functional prototype that you can iterate on. If the cassette deck is too bulky, you could use a modern analog playback module with a headphone jack and a pre‑amplifier instead. The key is keeping the timing deterministic so the image changes line up with the music.

Sounds like a solid plan. Just remember the deck’s sync is the backbone – keep the audio path clean and the MCU clock tight. If you drop a beep track, the MCU can lock onto that tone and use it as a hard‑coded beat. Good luck, and keep the design lean.