Q: Replace npc audio lines with 0ms silence

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Could anyone tell me if its possible to replace existing npc voiceovers played back with add_npc_line with a 0ms or very short silence?
I found some dummy files where the rest of the audio is, but could not find any reference to them (so how/if they are used).

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So best would be to add a new npc line with its own line id.
Can i pull it off without the lipsync file?