FLAC compression levels are (only) a trade of between encoding time and file size. The decoding time is pretty much independent of compression rate. In the following I will refer to the compression levels 0, ..., 8 as FLAC-0, ..., FLAC-8.
In short: I recommend FLAC-4!
The Easy Solutions
If I don't care about encoding time and since space is ...
I think what you are looking for is an expander, here's a link that explains it
Audio expansion means to expand the dynamic range of a signal. It is basically the opposite of audio compression.
Like compressors and limiters, an audio expander has an adjustable threshold and ratio. Whereas compression and limiting take effect whenever the ...