What is the link between Usages and the Audio HAL?

I am trying to get audio working on a board to which we're porting Android.

Currently, sounds played with usages such as USAGE_ALARM (4) are audible, whereas sounds played with usages such as USAGE_MEDIA (1) are silent.

For the audible usages, we can see calls to the Audio HAL:

...
D AudioFlinger: Client defaulted notificationFrames to 11025 for frameCount 22050
D AF::TrackHandle: OpPlayAudio: track:64 usage:4 not muted
D audio_hw_primary: out_set_parameters: enter: kvpairs: routing=2
D audio_hw_primary: out_set_parameters: exit: code(0)
I audio_hw_primary: start_output_stream_primary... 0xf3856000, device 2, address , mode 0
I audio_hw_primary: select_output_device(), headphone 0 ,headset 0 ,speaker 2, earpiece 0,
I audio_hw_primary: get_card_for_device adev: 0xf3828000, device: 2, flag: 0, card_index: 0xf3856158
W audio_hw_primary: card 0, port 0 device 0x2
W audio_hw_primary: rate 48000, channel 2 period_size 0xc0
W StreamHAL: Error from HAL stream in function get_presentation_position: Operation not permitted
...

Whereas for silent usages, we see only:

...
D AudioFlinger: Client defaulted notificationFrames to 11025 for frameCount 22050
D AF::TrackHandle: OpPlayAudio: track:65 usage:1 not muted
W StreamHAL: Error from HAL stream in function get_presentation_position: Operation not permitted
...

I thought that /vendor/etc/audio_policy_configuration.xml might be important. I've paired it back to just having one output, but nothing has changed.

        <attachedDevices>
            <item>Speaker</item>
        ...
        <defaultOutputDevice>Speaker</defaultOutputDevice>
        <mixPorts>
            <mixPort name="primary output" role="source" flags="AUDIO_OUTPUT_FLAG_PRIMARY">
                <profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
                         samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>
        ...
        <devicePorts>
            <devicePort tagName="Speaker" type="AUDIO_DEVICE_OUT_SPEAKER" role="sink" >
            </devicePort>
        ...
        <routes>
            <route type="mix" sink="Speaker"
                   sources="primary output"/>

What part of AOSP is responsible for the routing of different usages?

Why do some work and other are silent?

from What is the link between Usages and the Audio HAL?