when I recognized the voice of Tom.是时间状语从句。

如果改为 that I recognized the voice of Tom.是原因状语从句。（从意思方面理解的）

又如：

I was very glad when the boys won the relay race. 男同学们赢得了接力赛跑时,我非常高兴。（时间状语从句）

I am glad that the buyers have accepted my opinion. 我很高兴买主已经接受我的意见。（原因状语从句）