By default OBS is not playing back audio what it is captures. If you need to hear in real-time what is captured right now, then you need to use "Audio Monitoring" feature of OBS. The "Audio Monitoring" able to output sound not only to the main output (file or stream) but also to standalone device (usually headphones or PC speakers). The main menu Edit > Advanced Audio Properties opens window where you can specify which source of audio you want to monitor. Device through which you can hear specified audio source can be set in OBS main menu File > Settings > Audio > Advanced (group) > Monitoring Device. If you are using monitoring feature, make sure it is NOT set to "Default", because recently plugged device may become the default one without your knowledge (OBS don't has indication in the UI that the device for audio monitoring was changed recently or not available anymore).
Usually, during VHS capture, you don't need to monitor the sound. You seeing that sound is active in Audio Mixer pane (level-meter moves and is not grayed out), and the sound automatically outputs to the file. You can playback the recording later by VLC Player or mpv player.
Edit: of course, monitoring through the "PC speakers" while capturing these speakers is not wise. OBS will try to prevent this whenever possible.