In a universe where 'aether-sonis' permeates space as a massless, invisible medium capable of transmitting vibrations, the experience of a nearby spaceship witnessing an explosion would diverge significantly from real-world physics. Here's the breakdown:

### **Visual Experience**  
The crew would first observe a blinding flash of light from the explosion. This electromagnetic radiation (visible light, X-rays, etc.) travels at the **speed of light (c ≈ 3 × 10⁸ m/s)** in a vacuum, regardless of the aether-sonis. The visual effects—such as the expanding fireball, debris, or radiation—would unfold over time, depending on the explosion's scale and proximity.

### **Auditory Experience**  
The explosion would generate vibrations in the aether-sonis, propagating as **sound waves**. The speed of these waves depends on the aether-sonis' properties. Since it is massless and "perfectly transmits vibrations," we might assume it has **extremely high stiffness** (like a rigid solid) to minimize resistance to compression. However, without mass, inertia is absent, which in classical physics would prevent sound transmission. For the sake of the scenario, we’ll posit that the aether-sonis allows sound to travel at a **significant fraction of the speed of light**, though still slower than light itself (as no known or hypothetical medium transmits sound at *c*).  

For example, if sound in aether-sonis travels at **1% the speed of light (~3 × 10⁶ m/s)**, a ship 1 kilometer away would hear the explosion **~0.3 milliseconds after seeing it** (compared to light’s 3-microsecond travel time). At greater battle distances (e.g., 1,000 km), the delay would stretch to ~0.3 seconds. The sound might resemble a deep, resonant boom or rumble, depending on the explosion’s frequency content and the aether’s acoustic properties.

### **Simultaneity of Sight and Sound**  
- **If the aether-sonis transmits sound at relativistic speeds (e.g., >90% of *c*)**: The crew might perceive the flash and sound **nearly simultaneously**, with a negligible delay. This would create a dramatic, unified sensory experience akin to Earth’s thunderstorms, where lightning and thunder seem simultaneous at close range.  
- **If the speed of sound is significantly slower (e.g., <1% of *c*)**: The crew would see the explosion **first**, followed by the sound after a noticeable delay. This mirrors real-world explosions, where light arrives faster than sound in air.  

### **Physics Considerations**  
1. **Energy Transfer**: The explosion’s energy would split between electromagnetic radiation (light) and mechanical vibrations (sound in aether-sonis). The ship might also experience physical effects, such as radiation heating or debris impacts.  
2. **Sound Transmission**: For the crew to hear the explosion, the aether-sonis’ vibrations would need to couple with the ship’s hull, causing it to vibrate and transmit sound into the cockpit’s air or structure.  
3. **Aether-Sonis’ Contradictions**: A massless medium defies classical physics, as sound typically requires both elasticity and inertia (mass). This paradox could be handwaved as an exotic quantum field or a relic of the universe’s fundamental structure.  

### **Conclusion**  
In most plausible interpretations, **sight and sound would not be simultaneous**. The crew would see the explosion instantaneously (at *c*), then hear it after a delay determined by the aether-sonis’ sound speed. If the medium transmits sound unusually fast, the two could merge into a cohesive sensory event. Either way, the existence of aether-sonis would revolutionize space warfare, making explosions audibly as well as visually catastrophic.