Odysseus's journey into Hades is really a look at Odysseus's feelings and beliefs.  In Hades he talks to several people who give us clues to things that occurred in the war, in Troy, and in Greek mythology.  All of these things reflect Odysseus in some way.  They reveal his past, present, and future and his beliefs.  I guess this could be considered a decent into Odysseus's mind for the reader because so much is revealed to us that helps us understand Odysseus better.  He talks to Tiresias, who tells Odysseus what his fate will be and why Poseidon is so angry with him.  He gives Odysseus a lot of good advice for the future of his journey, and tells him home to avoid future hardship and get home as fast as possible.  He also talks to his mother Anticleia who fills him in on how Troy is doing.  She died waiting for his return.  Odysseus then talks to many famous heroes that were in the war and in Greek myths.  They tell him things that ultimately and more importantly informs the reader on Greek history.  This information is more for the reader's benefit.  After interacting with many dead souls Odysseus gets frightened and runs to the ship.

The test of the Sirens represents the temptation that Odysseus has encountered along this journey, and how they have made him want to wander astray from the purpose of the journey.  His men hold him down though, and this represents how even though they have struggled to stay on task due to these temptations, they also realize the importance of the journey and getting back home, so they struggle through it and keep trudging on.  The Sirens themselves are all the distractions that Odysseus has faced, all the temptations he has come across to keep him from wanting to go home, such as Calypso.  Odysseus and his crew succeed in this test, and the Sirens fail because he did not come to their calls.  This success represents the intellect, desire, and persistence that Odysseus and his men have.  It shows that they know how important it is for them to go home.