Is it possible to play 2 sounds simultaneously?
For example: one via HDMI (audio from a video) and another one (custom audio, wav, or whatever) via analog output at the same time?
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The Raspberry Pi can only use one output. This is by hardware design and is to do with DMA (Direct Memory Access)
You can plug in a USB audio device then you can use two output sounds at the same time.
The next best thing could be to play a sound out on HDMI, switch to Analog and play a sound there. But not at the same time and switching could make strange noises on both sources.
It's possible (at least on Raspberry Pi 3). I have Kodi playing audio through HDMI and MPD playing audio through the analogue output. They can do so simultaneously. See my answer to a similar question.
In short, send one sound to card 0 device 0 (analogue) and another to card 0 device 1 (HDMI).