Since the speakers currently work fine over Bluetooth with your iPhone, assume for now that the problem is on the iPod side of the connection. And you're also saying that the iPod did work properly with the speakers initially, just recently.
In case the iPhone is somehow interfering, turn OFF the iPhone while testing with iPod. Unplug the speakers from power (or do whatever you think is appropriate to "reset" them), and then reconnect to power. Do a Reset (restart) on the iPod
Learn how to reset your iPod - Apple Support
Follow the directions in the user guide (starting on page 53)
https://manuals.info.apple.com/MANUALS/1000/MA1624/en_US/iPod_nano_2015_user_gui de.pdf
to make the speakers discoverable and pair with iPod. Does the "low sound" problem still occur?
If so, in the iPod's onboard Settings, select the command to Reset Settings. This resets the iPod's onboard settings to default (does not affect data). Then try the speakers again. If the problem recurs, you may want to do a Restore on the iPod using iTunes. This erases the iPod, reinstalls its software, and sets it to default settings
Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support
Then, after syncing back your songs, try the speakers again.