They are never going to sound like tower speakers and as Mac Jim pointed out they are reasonably loud with heavy base at full volume. Press the top or use hey siri to tell it to set volume to 100%, just in case it’s not. She will say “that’s very loud, are you sure”. You can say “set volume to 100% everywhere” to do them all or simply check on each one.
Additionally you can make sure in the Home App HomePod settings that reduce bass has not been enabled.
Note: the bigger HomePods are much louder than the minis.
Try moving them around to different areas or surface to recalibrate. They will sound louder if placed on solid wood. Use a solid surface, not wire stands.
if you have 5, you should set them up to be stereo paired. Only two of the same model can be paired, so you can have two sets of paired and the one extra. Place paired on each side of the room and select which is left or right.
Also any older music you might have does not sound as loud as music that was produced in recent years. For example the Beatles will not feel loud enough imo. A CD is much louder played through a proper sound system.