Do you have other sound-equipped locos and their volume is satisfactory?
Gong back to the units in question, is the volume too low for all the sounds (engine, horns, bell, etc.) or just for one specific sound? You also didn't say what exactly you have tried so far to increase the volume.
Did you check or try to adjust the CVs responsible vor master volume and the volume of individual sounds? Most decoders have that capability. Some decoders also have a physical potentiometer on the decoder which can be turned to adjust the master volume (like a volume control on a portable radio).
You should figure out what brand the decoder is (JMRI DecoderPRO should tell you that), then download a full manual for it to see what can be done about the volume.
If none of this works then maybe the speaker itself is the problem.