Final answer:
In troubleshooting software issues, a support agent would find an audio mixer least useful as it's not relevant to software diagnostics, unlike other utility tools like registry cleaners, disk defragmenters, or task managers.
Step-by-step explanation:
A support agent troubleshooting problems with application software would likely find little use for an Audio mixer. The other utility tools listed are more closely related to system performance and stability. A registry cleaner can help fix errors in the Windows registry, a disk defragmenter can optimize data layout on the disk for better performance, and the task manager provides insight into running applications and system resource usage. However, an audio mixer is generally used for managing audio input and output levels and is not a troubleshooting tool for software issues. Hence, an audio mixer is the utility tool you would least expect a support agent to use when solving problems with application software.