If your employer wants to introduce continuous audio recording, you should at least be consulted properly.
The Information Commissioner's updated CCTV Code of Practice does not refer specifically to audio recording in HGV cabs, but it does acknowledge that audio recording, especially continuous, intrudes more on privacy in most situations than visual recording and will therefore require greater justification by the employer.
Continuous recording in an HGV cab is a serious intrusion on privacy, as drivers spend many hours in their cab, so it needs persuasive justification. Your employer should produce proper evidence of a real and pressing need (such as road safety) and should show that audio recording is a sensible way to address this need.
To comply with the Code, your employer should conduct a proper privacy impact assessment, share the results with drivers, consult fully, listen to drivers' concerns, consider alternative solutions and make reasonable modifications.
Your employer should think in terms of 'privacy by design'. In other words, when ordering technology that invades workers’ privacy, they should choose the version that performs the functions they need with the lowest possible privacy impact.
At the very least, drivers should be able to turn off the audio setting when they are parked up and taking a break.