The Idea in Brief
The Greek verb ἀκούω means to hear and/or to understand.
In the Greek New Testament, the verb ἀκούω means to hear and/or to understand. For example, in the following verse people can "hear" the voice but they cannot "understand" what the voice is saying.
1 Cor 14:2 (mGNT)
2 ὁ γὰρ λαλῶν γλώσσῃ οὐκ ἀνθρώποις λαλεῖ ἀλλὰ θεῷ, οὐδεὶς γὰρ ἀκούει, πνεύματι δὲ λαλεῖ μυστήρια:
1 Cor 14:2 (NASB)
2 For one who speaks in a tongue does not speak to men but to God; for no one understands, but in his spirit he speaks mysteries.
In this verse, the verb ἀκούω is translated "understands" notwithstanding that the voice is audible. Thus the idea of ἀκούω not only includes "hearing" but may also include actual understanding of what is heard. So the best translation of the relevant two verses in the Book of Acts (following these guidelines for ἀκούω) would be as follows.
Acts 9:7 (NASB)
7 The men who traveled with him stood speechless, hearing the voice but seeing no one.
Acts 22:9 (NASB)
9 And those who were with me saw the light, to be sure, but did not understand the voice of the One who was speaking to me.
In conclusion, the Greek verb ἀκούω means to hear and/or to understand. The nuance is also evident in modern colloquial English (viz., "Did you not hear me!?"). The translators of the NASB captured this nuance, and therefore the NASB provides the best translation of these passages in the Book of Acts.