Sounds are pressure waves in air, but the inner ear is a liquid-filled space. This presents an impedance matching problem where sound is reflected rather than transmitted.

The eardrum and inner ear bones perform this mechanical impedance matching/transduction of air-to-liquid sound. From Purves' Neuroscience:

Sounds impinging on the external ear are airborne; however, the environment within the inner ear, where the sound-induced vibrations are converted to neural impulses, is aqueous. The major function of the middle ear is to match relatively low-impedance airborne sounds to the higher-impedance fluid of the inner ear. The term “impedance” in this context describes a medium's resistance to movement. Normally, when sound travels from a low-impedance medium like air to a much higher-impedance medium like water, almost all (more than 99.9%) of the acoustical energy is reflected. The middle ear (see Figure 13.3) overcomes this problem and ensures transmission of the sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost 200-fold by the time it reaches the inner ear.

Some sources will refer to this as "amplification" which is correct in some ways (without it, sound would be too "weak" in the inner ear) but doesn't quite explain the whole problem.

Inner hair cells do the actual sensory transduction, converting vibrations into electrical signals when the "hairs" are stretched apart mechanically, causing a fragile "tip link" to pull on a physical channel, opening it and allowing ions to flow through. The cochlea is a frequency-analyzer, vibrating at different frequencies along its length and allowing different hair cells to respond maximally to different frequencies of sound.

Sounds are pressure waves in air, but the inner ear is a liquid-filled space. This presents an impedance matching problem where sound is reflected rather than transmitted.

The eardrum and inner ear bones perform this mechanical impedance matching/transduction of air-to-liquid sound. From Purves' Neuroscience:

Sounds impinging on the external ear are airborne; however, the environment within the inner ear, where the sound-induced vibrations are converted to neural impulses, is aqueous. The major function of the middle ear is to match relatively low-impedance airborne sounds to the higher-impedance fluid of the inner ear. The term “impedance” in this context describes a medium's resistance to movement. Normally, when sound travels from a low-impedance medium like air to a much higher-impedance medium like water, almost all (more than 99.9%) of the acoustical energy is reflected. The middle ear (see Figure 13.3) overcomes this problem and ensures transmission of the sound energy across the air-fluid boundary by boosting the pressure measured at the tympanic membrane almost 200-fold by the time it reaches the inner ear.

Some sources will refer to this as "amplification" which is correct in some ways (without it, sound would be too "weak" in the inner ear) but doesn't quite explain the whole problem.

Inner hair cells do the actual sensory transduction, converting vibrations into electrical signals when the "hairs" are stretched apart mechanically, causing a fragile "tip link" to pull on a physical channel, opening it and allowing ions to flow through. The cochlea is a frequency-analyzer, vibrating at different frequencies along its length and allowing different hair cells to respond maximally to different frequencies of sound.

As a side note, although you asked about vertebrates, aquatic vertebrates like fish don't have these middle ear bones, their hearing is quite different from the mammalian ears I am most familiar with. I'm not sure at all about hearing mechanisms in aquatic mammals that evolved from terrestrial ancestors, like whales and dolphins, but that might be a good subject of a later question.