Being one of the large 'ALL' islands, this one Still does not seem to be the most difficult, because modern composer differences sound more pronounced than for example Romantic. [All Modern]

Compare russian composer Glazunov to his russian student Shostakovich.

Glazunov sounds more academic and old-fashioned.

Shostakovich sounds more dissonant, abrupt, fugue, dark, tragic.

Only Glazunov has violin concerto in this game.

Only Shostakovich has cello concerto, piano, vocal in this game. (Glazunov, Shostakovich)

French late-romantic Fauré sounds more romantic, delicate, light, naive.

Russian modern Shostakovich sounds tragic, bitter, cold, dissonant.

Beware of Fauré Pelleas et Melisandre, which sounds wide and deep.

Beware of Shostakovich fugues, which sound baroque.

Only Shostakovich has cello concerto in this game.

Only Fauré has piano+solo string, piano song in this game.

Use language to guess vocal pieces. (Fauré, Shostakovich)

Beware of Shostakovich Gadfly suite, which sounds very simple and romantic. (Borodin, Shostakovich)

Only Shostakovich has piano, cello concerto in this game.

Only Sibelius has violin concerto in this game.

Shostakovich sounds more dissonant, abrupt, fugue, dark, tragic.

Sibelius sounds more simple, folk, sometimes dancing, but often broad.

Use language to guess vocal pieces. (Shostakovich, Sibelius)

This is an interesting island with three composers of different periods.

Debussy is french impressionist with hazy mistical sound.

Rachmaninov is russian post-romantic with simple light melodies, piano bell chords and fast piano textures.

Shostakovich is russian modern with tragic, dissonant, abrupt, fugue, dark sound. (Debussy, Rachmaninov, Shostakovich)

This is an easy island, Shostakovich being much more consonant and Berg being 12-tone and chaotic.

This is interesting, that Shostakovich obviously has some dissonances and this often helps to distinguish him from romantic and post-romantic composers. But on this island he sounds much more consonant.

By the way, comparing Berg to Shostakovich seems more difficult to comparing Webern to Shostakovich, because Berg often sounds more romantic. (Berg, Shostakovich)

This is an easy island, Shostakovich being much more consonant and Webern being 12-tone and chaotic. Watch out for Webern Two Early Pieces for Cello and Piano, Im Sommerwind, Langsamer Satz, Passacaglia, which sound very romantic.

This is interesting, that Shostakovich obviously has some dissonances and this often helps to distinguish him from romantic and post-romantic composers. But on this island he sounds much more consonant.

You can use language to distinguish vocal pieces safely. (Shostakovich, Webern)

These composers are a century apart, belonging to non-adjacent periods, but their symphonic pieces can be not easy to guess at the first time, because romantic french Berlioz is progressive, while modern russian Shostakovich often sounds traditional.

Piano - only Shostakovich.

Vocal - you can use language to guess.

Berlioz sounds more romantic, repetitive, melodic.

Shostakovich sounds more dissonant, abrupt, fugue, dark, tragic. (Berlioz, Shostakovich)

This is an interesting comparison of romantic french Berlioz, post-romantic english Elgar and modern russian Shostakovich.

Please see comments to islands comparing pairs of these composers. (Berlioz, Elgar, Shostakovich)

Compare late romantic english Elgar to modern russian Shostakovich. Elgar does not have solo piano on this island, but beware the solo piano parts of Elgar's Sea pictures. Only Shostakovich has piano concerto in this game. Shostakovich is more complex, tragic, dissonant, melodic, fugue. Elgar is more consonant, rhythmic. Both have cello concertos. Do not repeat my mistake and think that Elgar will sound like simple romantic in this comparison. (Elgar, Shostakovich)

Gershwin should be no problem to guess. While two others are both russian and both can sound wide and melodic at times. Lighter pieces with pizzicato and melodic percussion are more common for Tchaikovsky, while Shostakovich usually sounds more serious and depressive. (Gershwin, Shostakovich, Tchaikovsky)