Q:

What is playing on Shinji's walkman? 

Shinji's Walkman (I don't know the exact term in English, but I already know it's a rare cassette player that didn't take off except in Japan), but what I'm interested is the music he is listening on it. ( More specifically, the cassette have 26 tracks, and a duration of around 1h30 if I trust my memory of the appearance of the player). 
I've re-watched NGE and the RoE:1.01, and the music have a great importance on Shinji's development, but we do not hear enough for any app to recognize it, and the name don't seems to be revealed in the credits. 
Did the name of the cassette/compilation/music Shinji is listening to revealed in the show, a interview or any other "official" source? 

A:

Take a look at this cut from Episode 4 (via the official storyboards), and also the Shinji Ikari character page notes that the two songs that play are:

The two songs usually played on Shinji's SDAT are: Track 25 for "You are the only one" and Track 26 for "Blue Legend", respectively.  The songs are from "Lilia from Ys", an image album for the "Ys" RPG series.  The Vocalist is Kotono Mitsuishi, Misato's seiyu.  However, Episode 02 has two Eva BGM pieces playing, while Episode 23 has a different vocal song for Track 25 instead.  

Also on Youtube:
Aoi Legend, You are the only one

A:

This article on EvaWiki has all the tracks played by Shinji's walkman/SDAT. 
http://wiki.evageeks.org/SDAT
Edit:
Episode 2 - A-2 ("Background Music" - S² Works, Disc-01, track 02) and B-4 ("Fly Me To the Moon" - S² Works, Disc-01, track 27)
Episode 4 - "Blue Legend" (蒼いレジェンド Aoi Legend), track 3 of Lilia ~from Ys~ and "You are the only one"[5], track 1 of Lilia ~from Ys~
Episode 9 - "Blue Legend" (蒼いレジェンド Aoi Legend) and "Distant Empty Promise"[6] (遠い空の約束 Tôi Sora no Yakusoku), track 8 of Lilia ~from Ys~
Episode 15 - "You are the only one", track 1 of Lilia ~from Ys~
Episode 23 - "Ryoute Ippai no Yume" (両手いっぱいの夢 lit.  Both Hands Full of Dreams), track 4 of Masami Okui ‎- Gyuu
Episode 24 - Ode to Joy - Beethoven
Episode 25' - It plays nothing...
If you would like to know more there are
Music in Chronological Order of Usage and FLY ME TO THE MOON Guide

