He is asking if the dialogue can be extracted in isolation. That is, without any music (or sound effects).
Yes. As long as he sets the music setting to MIDI, he can record the dialogue. Sound effects cannot be excluded. Though that shouldn't be a problem to fix afterwards since they rarely occur simultanelously with dialogue. The sound you hear as you choose an option in a dialogue is MIDI based and won't be recorded with the dialogue.
If he however choose an exterior recorder, outside of DosBox, he will be have to set the music setup to "No music" if it exists (I can't remember) since those programs most likely will not be able to differentiate between MIDI and sound.
I believe the command for recording sounds to WAVE in DosBox is Ctrl + F6.