pactl: command not found

Installed with PulseAudio are the PulseAudio utilities (pulseaudio-utils package). These are command-line utilities for managing PulseAudio and playing sound files. The paplay and pacat will play sound files; pactl will let you control the sound server; and pacmd lets you reconfigure it. Check the man pages for each for more details.

pactl or pacmd, these two commands do basically the same thing. pacmd is the interactive version with more options. For example, pacmd with the command list-sinks includes the following:

name: [alsa_output.pci-0000_00_1b.0.analog-stereo]
driver: [module-alsa-card.c]
flags: HARDWARE HW_MUTE_CTRL HW_VOLUME_CTRL DECIBEL_VOLUME LATENCY FLAT_VOLUME DYNAMIC_LATENCY
state: SUSPENDED
suspend cause: IDLE
priority: 9959
volume: 0:  93% 1:  93%
        0: -1.88 dB 1: -1.88 dB
        balance 0.00
base volume: 100%
             0.00 dB
volume steps: 65537
...

Using pactl, you can load the module module-loopback with this:

$ pactl load-module module-loopback latency_msec=1

When the module is loaded, sound is internally routed from the input device to the output device. The latency is effectively zero.

In case you encounter the below error:

pactl: command not found

You may try installing below package as per your choice of distribution.