Update: Motorola pinged us with an official response. Check it out after the break.
Motorola Mobility is aware of a very small number of ATRIX 4G users who have reported low audio issues when placing voice calls. Motorola ATRIX 4G is designed and tested to meet our high standards for audio quality. As part of our longstanding commitment to delivering high quality products that our customers love, we believe in reacting even when identified issues impact only a handful of users. Motorola is evaluating further optimization of audio levels in a future maintenance release. Any consumer who experiences this issue should contact a Motorola customer support center.
Motorola takes great care to ensure devices are developed with quality in mind. We appreciate consumer feedback and will continue working to refine our product experience.