The following code can be downloaded directly to your Raspberry Pi. It will read the trimpot value, translate the reading to a volume range and modify the OS output volume level on your Raspberry Pi. The remap_range() method is being used to convert the 16-bit analog in range 0 - 65,535 to volume 0-100%.

analog.py

import os
import time
import busio
import digitalio
import board
import adafruit_mcp3xxx.mcp3008 as MCP
from adafruit_mcp3xxx.analog_in import AnalogIn

# create the spi bus
spi = busio.SPI(clock=board.SCK, MISO=board.MISO, MOSI=board.MOSI)

# create the cs (chip select)
cs = digitalio.DigitalInOut(board.D22)

# create the mcp object
mcp = MCP.MCP3008(spi, cs)

# create an analog input channel on pin 0
chan0 = AnalogIn(mcp, MCP.P0)

print('Raw ADC Value: ', chan0.value)
print('ADC Voltage: ' + str(chan0.voltage) + 'V')

last_read = 0       # this keeps track of the last potentiometer value
tolerance = 250     # to keep from being jittery we'll only change
                    # volume when the pot has moved a significant amount
                    # on a 16-bit ADC

def remap_range(value, left_min, left_max, right_min, right_max):
    # this remaps a value from original (left) range to new (right) range
    # Figure out how 'wide' each range is
    left_span = left_max - left_min
    right_span = right_max - right_min

    # Convert the left range into a 0-1 range (int)
    valueScaled = int(value - left_min) / int(left_span)

    # Convert the 0-1 range into a value in the right range.
    return int(right_min + (valueScaled * right_span))

while True:
    # we'll assume that the pot didn't move
    trim_pot_changed = False

    # read the analog pin
    trim_pot = chan0.value

    # how much has it changed since the last read?
    pot_adjust = abs(trim_pot - last_read)

    if pot_adjust > tolerance:
        trim_pot_changed = True

    if trim_pot_changed:
        # convert 16bit adc0 (0-65535) trim pot read into 0-100 volume level
        set_volume = remap_range(trim_pot, 0, 65535, 0, 100)

        # set OS volume playback volume
        print('Volume = {volume}%' .format(volume = set_volume))
        set_vol_cmd = 'sudo amixer cset numid=1 -- {volume}% > /dev/null' \
        .format(volume = set_volume)
        os.system(set_vol_cmd)

        # save the potentiometer reading for the next loop
        last_read = trim_pot

    # hang out and do nothing for a half second
    time.sleep(0.5)