This is caused by the sound effect called “fade-in music”. External speakers or headsets sometimes do this – when a new sound starts, they assume it’s a start of new song and gradually increase volume from 0 to whatever volume level you have set. Sometimes people also install apps that process sound coming out of the device to produce different sound effects, one of them may be “fade-in music” (other effects may be echo, concert hall emulation etc.). Please disable such sound processing apps when you listen to speech, or if it’s on your external speaker or headset – ask the support personnel of that device how to disable “fade-in music” effect. All of this cannot be controlled from the level of @Voice app program code.