Audacity: save just a sound file, not a Project

Or is there a lightweight tool better suited to such tasks, on either Windows or Linux?

On Windows, there is WaveShop. It's similar to Audacity and the save process is shorter with 3 clicks:

1. Close WaveShop via X
   • Prompt to save pops up
2. Click Yes
   • Prompt for MP3 quality settings pops up (don't change anything)
3. Click yes
   • WaveShop automatically saves it as mp3 and overwrites the original

Tadaa

The GUI is not the best looking one and there aren't as much options as Audacity has. So it's a bit more lightweight (7 MB vs 24 MB)

^{click to enlarge}

In Audacity, you can set keyboard shortcut to Export command (this will save you time opening file menu and hitting Export...) Also you can deselect the option "Show Metadata Editor prior to export step" in Preferences. This will make it a bit less painful (sorry for my sarcasm)