Trace audio from application to card?

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This is primarily for study. 

I'm wondering if there's a way to logically trace the flow of audio through the Alsa stack?

I.e. - generically I run some app that creates sound. Possibly it's complicated like YouTube in Chrome, or something easier like aplay. Is there any way to get an overview of how that sound flowed locally? Did it go directly to a hardware device? Did it go through dmix or pulseaudio? If I thought I was sending audio to a specific device, say HDMI on a graphics adapter to go to an external mixer or a TV but I didn't hear the audio is there a way to create some sort of logical flow chart - even if it's just a list of apps and hardware - to show what Alsa believes is happening?

An example:

1) I run aplay sample1.wav in a terminal. I hear it.

2) I run aplay sample2.wav in a second terminal. I hear it.

3) I run both samples in different terminals at the same time and I hear both.

In the case of #3 maybe the audio automatically turned on dmix to create a single mixed audio whereas possibly #1 & #2 didn't use dmix? On my system I use pulseaudio for the internal sound card so I suspect it was mixed in pulseaudio but can this be verified somehow?

In the case of my HDMI card I don't ever seem to hear anything no matter what hardware deive I put out on the HDMI cable. 

Thanks,
Mark
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