I've contacted the developer, so I can confirm that it should work:

The latest version of Replay Music can already do all those things.

To record from a line-in source you just need to go to Replay Music's settings and under the "Input" tab in the "Input Source" drop down list and select a line-in option. The options in this drop down list are dependent on the capability of your sound card.

Replay Music also has a normalize option that allows you to record all your audio to a constant volume. This setting is found under the "Output" tab in Replay Music's setting.

You can also normalize already recorded files by right clicking in the track list box and selecting "Import Mp3 files" and then selecting all the files you wish to import. Once the tracks are imported, select the files in the track list box and then right click and select the "Normalize Track(s)" option.