We present AudioMiXR, an augmented reality (AR) interface intended to assess how users manipulate virtual audio objects situated in their physical space using six degrees of freedom (6DoF) deployed on a head-mounted display (Apple Vision Pro) for 3D sound design. Existing tools for 3D sound design are typically constrained to desktop displays, which may limit spatial awareness of mixing within the execution environment. Utilizing an XR HMD to create soundscapes may provide a real-time test environment for 3D sound design, as modern HMDs can provide precise spatial localization assisted by cross-modal interactions. However, there is no research on design guidelines specific to sound design with 6DoF in XR. To provide a first step toward identifying design-related research directions in this space, we conducted an exploratory study where we recruited 27 participants, consisting of expert and non-expert sound designers. The goal was to assess design lessons that can be used to inform future research venues in 3D sound design. We ran a within-subjects study where users designed both a music and cinematic soundscapes. After thematically analyzing participant data, we constructed two design lessons: (1) Proprioception for AR Sound Design, and (2) Balancing Audio-Visual Modalities in AR GUIs. Additionally, we provide application domains that can benefit most from 6DoF sound design based on our results. To expand on these insights, we conducted a second within-subjects study comparing AudioMiXR to a 2D panner baseline. Results show that AudioMiXR significantly improved usability (SUS), reduced frustration and mental workload (NASA-TLX), and enhanced creativity across all subscales. These findings demonstrate that 6DoF AR interaction yields measurable gains in user experience and creative output, positioning AudioMiXR as a promising foundation for future AR-based sound design tools.