This seems to be a case where word play is lost in translation. As H3br3wHamm3r81 mentions, the Hebrew word קוֹל (kol) can be translated as both "voice" and "thunder." The nearness of both ideas when God speaks is seen in poetic parallelisms like 2 Samuel 22:14.
The Lord thundered from heaven;
the voice of the Most High resounded.
In other words, the Lord thundering and his voice resounding are nearly synonymous.
The surrounding context of also points towards an intentional ambiguity. Again, as H3br3wHamm3r81 notes, the adjacent word, הַלַּפִּידִם, often translated as lightnings, points us in the direction of "thunder". However, the fear of the people at the sight (sic) of the thunder/voices issues in the request found in the following verse that Moses speak to them rather than God. This would point in the direction of "voices".
Since the word itself involves an ambiguity, and since the context suggests an ambiguity, it seems best to see this as word play that cannot quite be capture in any one English translation.