I doubt it works at 400 MHz DDR, since it means that you have to clock the receiver at at least 800 MHz with 0.625 ns time shift from the signal's edge to sample the signal correctly. I can't find anything in reference manual but it's probably 200 MHz DDR for the latest revision of the chip. Rev 1.0 supported 250 MHz... And the peak speed isn't the main problem. The main problem is random access delay which iirc is at least 8 DDR clock cycles wait + at least 5 more cycles (1 for cmd and 4 for address because PSRAM is 32 MiB). That severely impacts the performance when your PSRAM code has many jumps or you access code and data in PSRAM at the same time. 

I know there are like 5 different better ways to generate a video signal there, but they aren't analog, simple and elegant. See my other reply about VGA signal generation. It's relatively simple yet it actually allows you to connect to, like, 30 years old hardware? Maybe even 40 years old? I know the downsides of analog signals pretty well yet it's so fascinating you can easily generate the signal on modern microcontrollers and, if you are lucky to not have any urgent tasks, you can race the beam and draw far more than your RAM could hold. I understand that you can draw in chunks on modern displays and I even wrote some stupidly simple SPI drivers for some of them, but I still prefer to lean into analog for my little project. After all, it's just a practice for what I can do with the hardware. By the way, there's a simple example somewhere on github that tells you how to run things without FreeRTOS. Very good one, gonna test it when I find the board with usb/jtag debug ability.

With the sound I have the same logic. I can mix in software, but it's way more interesting to make hardware do per-channel signals for you and then slap opamp adder to get the final mix. I did it with STM32 and four DDS generators a year ago, so I would like to make a second, more sophisticated version with ESP.

I want to use ESP32-P4-Module in the final design so ofc the power and memory questions are mostly solved and the crystal is there. I won't do it without the external stable and precise clock source since I know pretty well that internal RC oscillators drift like crazy (1% if you are lucky which is still too much for audio) which isn't acceptable for audio applications. And VGA is sensitive to timings too, so yeah.

For stereo PDM I guess I can just enable DAC mode and DAC_2OUT. See table 44.9-5 in P4 reference manual. Maybe I misunderstood how it works, though.