The possibility of steady-state population inversion in a small sample of strongly driven two-level emitters such as quantum dots (QDs) in microcavities, and its utilization towards amplification of light and acoustic waves, is investigated theoretically. We find that inversion and absorption spectra of photons and phonons crucially depend on the interplay between the intrinsic vacuum and phonon environments. The absorption profiles of photons and phonons show marked novel features such as gain instead of transparency and absorption reversed to gain, respectively. Furthermore, we report collectivity-induced substantial enhancement of inversion and pronounced gain in the photon and phonon absorption spectra for a subwavelength-sized QD ensemble.