You'll need to check your pc's audio device settings and accompanying manual (motherboard manual if on-board audio) to see if the default action of plugging something into the front audio jack automatically disables some or all other audio outputs e.g. line out, speaker out. Many do behave like this by default, though most recent audio panels will allow the user to re-configure any socket via an automatic pop-up box immediately when something's plugged in.
Ultimately, your audio device will dictate what you can and can't do. Through my own on-board Realtek HD Audio Manager (icon in taskbar) I have a Device Advanced Settings page where I can mute/unmute the outputs when something's plugged into the front headphone (mute by default). Check for something like this.