horror game really relies on the mood and atmosphere, you nailed it on the visual and audio. And the introduction of different audio and weird creatures really make it horrifying.
For the visual: I would recommend adding some flickering visual, such as a flickering ceiling light or table lamb. The weather maybe rainy and sometimes thunder come out. frightening creatures can be added between the short time of lighting.
For the audio: I would really suggest an ambience sound (room tone) and other sound effects can be added to further build up the horror vibe. You can always hear those hollow and windy atmosphere sound continues to exist in the horror movie. Sometimes laughing sound, metal, high pitched, glass broken sound can build up the tension further more. These kind of technique would be kind of canned effect and stereotype, but it really produce instant effect within limited production time.