Virtual Reality (VR) applications with low-latency head tracking require high-quality spatial audio effects. However, classic VR/game sound approaches cannot properly simulate the acoustic of the real world. Current audio research is moving towards 3D spatial audio to have a more realistic simulation. In 3D spatial audio, the listener has the sensation that sound comes from a particular direction in 3D space. In other words, the listener can localize a source based on audio and have a more coherent and immersive experience when paired with visual simulation. In this new context, game engines should provide sound designers with a set of 3D spatial audio tools. The following common effects are desirable in this type of toolbox: reverberations and reflections, which can be employed in the creation of caverns or churches (places with lots of echoes); modulation, which can increase the perceived variety of a recorded sound, by slightly varying its pitch (as in the sounds of footsteps); mixing and fading volumes, which can create dramatic moments in storytelling and music reproduction. In this work, we propose a realtime audio engine to spatialize sound point sources in virtual environments. This engine is an open-source architecture that provides a basic set of audio effects and an efficient way to mix and match them. We implement 3D audio spatialization by leveraging recorded head-related impulse responses (HRIRs) and we produce special sound effects with digital signal processing (DSP) techniques. Although some powerful commercial audio SDKs for Virtual Reality are currently available (e.g. Oculus), our audio engine prototype may be a flexible option when adaptation, simplification, testing, and parameter tuning are necessary.