apologetic tone; defensive tone of voice
Describes a manner of speaking that sounds like one is making excuses or justifying oneself, often with a defensive or apologetic edge.
He explained the reason for his lateness in an apologetic tone.
When my boss pointed it out, I ended up replying in a defensive tone.
言い訳 is the excuse itself, while 弁明口調 describes the tone or manner of speaking that sounds like making excuses.
弁解 is a more formal justification or explanation, whereas 弁明口調 focuses on the apologetic or defensive quality of the voice.
Compound of 弁明 (explanation, justification) and 口調 (tone of voice).