The HomePod and the HomePod mini are designed to automatically adjust the volume of the Siri assistant based on the level of noise in the room, but you can also verbally adjust Siri's volume level separately from the volume level of the media that's playing.
This how to walks you through the commands that you need to specifically adjust Siri's volume as needed.
How to Change Siri's Volume on HomePod
On HomePod or HomePod mini, you can ask Siri to set the Siri volume to a specific percent, which is an adjustment that is separate from the media volume.
To do so, just say "Hey Siri, change your volume to [percent]." You can also say "Hey Siri, speak at [percent]."
If you tell Siri to set the Siri volume to 34 percent, for example, it will adjust it to that level while the music volume stays the same. Siri can be forced to be quieter or louder than your music.
Change the Media Volume on HomePod
You can also verbally adjust the media volume with a similar command, and this will adjust just the song volume without changing the Siri sound level.
To change the media volume, just say "Hey Siri, change the media volume to [percent]."
Music levels can also be adjusted using the - and + buttons on the top of the HomePod or through a connected iPhone or iPad, but adjusting Siri volume specifically seems to require the spoken command.
Update: Multiple MacRumors forum members have reported that Siri's volume does not stay at the requested level permanently, and reverts back to a default automatic level anywhere from minutes to hours after the request. There is, unfortunately, no known fix for the issue at this time.