Headphone Amp for Raspberry Pi only working with Pulse, not with ALSA

Support
1

When following the guide I can successfully enable audio output for example with mpg123 -o pulse . Without -o something else is chosen and gives garbled output rendering much too fast – a 3min MP3 is finished within <30s. aplay -L shows the correct list of devices and aplay gives clear output. (Raspbian Buster)

mp3blaster works right away with the same MP3-files, didn’t check on which device/ through which library.

aplay -L:

null
    Discard all samples (playback) or generate zero samples (capture)
jack
    JACK Audio Connection Kit
pulse
    PulseAudio Sound Server
default
    Playback/recording through the PulseAudio sound server
sysdefault:CARD=sndrpihifiberry
    snd_rpi_hifiberry_dac, 
    Default Audio Device
dmix:CARD=sndrpihifiberry,DEV=0
    snd_rpi_hifiberry_dac, 
    Direct sample mixing device
dsnoop:CARD=sndrpihifiberry,DEV=0
    snd_rpi_hifiberry_dac, 
    Direct sample snooping device
hw:CARD=sndrpihifiberry,DEV=0
    snd_rpi_hifiberry_dac, 
    Direct hardware device without any conversions
plughw:CARD=sndrpihifiberry,DEV=0
    snd_rpi_hifiberry_dac, 
    Hardware device with all software conversions
usbstream:CARD=sndrpihifiberry
    snd_rpi_hifiberry_dac
    USB Stream Output