Hey there, Neptue78.
Welcome to the Apple Support Communities! Thanks for posting about audio levels on your AirPods when used with a transmitter. We'd be glad to see what we may be able to do to assist.
When you use your AirPods with an iPhone, do you have the same behavior, or does the lower audio level only appear when utilizing the external Bluetooth transmitter? If the issue is specific to using that transmitter, you may want to reach out to the manufacturer of the transmitter to inquire about potential firmware/software updates or additional steps to take to normalize the audio levels. If the audio is low on your iPhone as well, Reset your AirPods to see if that changes anything and let us know what you find out.
Take care.