I was once handed a "professionally" shot video that had not turned out well. Two high-end cameras' microphones had failed to work, so all we had to work with was audio picked up by two home-video type handheld camcorders meant to be safety backups. Their video quality was OK because the lighting was good, but the audio was horribly compressed, while noise was amplified by the cameras' AGC circuits.
I did not have access to any restoration software such as Rx, but was able to employ two types of processors to help. First was a transient designer. At the time all I had was Waves' TransX. I used the multi-band version and concentrated on the upper-frequency bands. Today I'd try Meldaproductions' MTransientMB first.
Second tool was an expander. Back then all I had was the Sonitus Compressor with a negative ratio. Today I'd turn to Meldaproduction's
MDynamics plugin, which lets you define your own transfer curve (as well as having just about every other feature a compressor can have).
Other plugins to consider (caveat: I don't have experience with all of these):
Fabfilter's Pro-G
Hornet CompExp (free)
BT ExpanderGate (bundled with SONAR)
The Dynamics module in Ozone Advanced
DMG Compassion
Flux Pure Expander (requires dongle)
Flux Solera (requires dongle)