Kernel Task without Task

A runaway kernel_task can be due to a number of factors, such as:

• a failed or disconnected temperature sensor
• cooling system inefficiency (an inoperative exhaust fan on Macs so equipped, possibly clogged with dust, including the heat sink)
• a worn out, missing, or non-Apple (user-installed) battery on portable Macs
• similar hardware-specific causes including internal hardware additions or modifications, external USB and other connected devices, or the driver software required to use them.

If you can find no other explanation for its behavior, an SMC Reset is justified.

Given the additional details regarding an inability to select that Mac's internal speakers suggests a hardware fault may be the cause. Consider running Apple Diagnostics: Diagnose problems on Mac - Apple Support, although only two outcomes are possible: it will either report "no trouble found" when trouble clearly exists, or it will identify a specific fault that only Apple would be able to fix anyway. If you suspect that's the cause, then contact Apple using the Support link below.

Please read If kernel_task is using a large percentage of your Mac CPU - Apple Support.

If nothing seems to help, please write back. Before you do though, remove "OnyX". Uninstall it. Using it or similarly categorized utilities to "clean" a Mac only cause trouble. Don't use such things.