I remember ETI published on back in the 1970's. It was project ETI484.
Compression and expansion, ratio up to 2:1 with fixed attack and slow/fast release. Fairly simple and made to help with noise reduction on cassettes.
There are probably other projects around, it was quite popular back then.
There are also opto compressors, VCA compressors and FET compressors used in pro audio. There are quite a few circuits around for these.
It sounds like you only want compression, and only one channel, so it should be fairly simple to build.