Top products from r/ShigatsuwaKiminoUso

Here you go. http://anime.thehylia.com/soundtracks/album/shigatsu-wa-kimi-no-uso-ed-single-kirameki Please consider purchasing the album to support the artists! Link to buy.

To be perfectly honest I've never heard the specific "fast" version you posted, with the female singer. No idea where that is from! The "fast" version here is the one with the male singer from the anime's first TV ending (but it's the full song). Pretty sure this version is the "original"/canonical Kirameki.

The slow version (both your link and mine) is the Kaori & Kousei performance version from the second half of the last episode -- again featuring the same male singer.

Cheers!

The Boku to Kimi to no Ongakuchou album contains the vast majority of the performances you're looking for.

Some others, like Mozart's Twelve Variations on "Ah vous dirai-je, Maman" and Tchaikovsky's Rose Adagio from The Sleeping Beauty, can be found on the Twinkle Little Star album.

Masaru Yokoyama's arrangement of Chopin's Ballade No. 1 in G minor can be found on Yuna Shinohara's Estreno album.

The version of wacci's Kirameki that is played in the final episode can be found on the limited edition Kirameki album.

I may be wrong, but I believe there's only two discs.


Edit: YouTube disc 1


YouTube disc2

Unsure if there's more, but it's almost 3 hours combined.


Edit 2: Amazon link


Edit 3(last one!): I personally like disc 1 from the YouTube link, it's much more lively and "easier" to listen to IMO. Disc 2 is a bit more gloomy.

I think it's referencing one of two things. Maybe both:

1. Coda, a collection of manga short stories.
   
2. A Six Person Etude, a novel that takes place during Vol. 6 and 7 of the manga, or roughly between episodes 11 to 15, from the perspectives of Kaori, Watari, Tsubaki, Emi and Takashi.
   
   Those two things aren't hard to find to read online if you Google the right thing. 😉

Have you watched the OVA? Can't remember if we've discussed it before. It's the story of the piano competition that takes place before the show. There's also a collection of side stories for most of the characters but it's in manga form only. It's called A Six-Person Etude. I just ordered mine and can't wait to read it.

Link: https://www.amazon.com/gp/product/1945054263/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1


Edit: Adding to this there's another collection of short stories called Your Lie In April Coda/ Shigatsu Wa Kimi No Uso Coda, that I'm currently trying to find in physical form. It was a special inclusion with the first launch of the DVD.

You can buy the OST on Amazon.

And this is the album with the classical pieces.

On top of the answers already provided there are 2 more soundtracks.
[Shigatsu wa Kimi no Uso Boku to Kim to no Ongakuchou = all the performances in the show] (http://www.amazon.com/gp/product/B00N23RGU4/ref=pd_lpo_sbs_dp_ss_2?pf_rd_p=1944687702&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00TA3PZ8A&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0HEW8K1DDNJ99QC4V9CZ)
[Shigatsu Wa Kimi no Uso Twinkle Little Star = Other performances related to the show] (http://www.amazon.com/SHIGATSU-KIMI-TWINKLE-LITTLE-STAR/dp/B00TA3PZ8A)