You stated that your theme was that their love transcends death. Though the lovers were young when Annabel Lee died, his love for her has lived on past her death. Alliteration is an important factor in this poem, and I think the reason you may be having trouble is that you are looking mainly within the line or the next line to find the repeated sound. As the lines in Annabel Lee are short, the repeated sounds later in the stanza also figure in.
In the first stanza: repetition of “M” – many, many, maiden, maiden
In the second stanza: repetition of “L” – loved, love, Lee , love
In the third stanza: repetition of “S” –sea, so, shut, sepulcher, sea
In the fourth stanza: repetition of “H” – half, happy, heaven
In the fifth stanza: Repetition of “W” – we, who, wiser, we
In the sixth stanza: Repetition of “B” –bringing, beautiful, bright, beautiful
If the theme is eternal love, found in youth, but never forgotten, then part of the function of sound devices should be to remind us of that youthful love. Poe is a master of sound, and it is no accident that he reminds us of the youthful lovers by repeating so many short words (generally no longer than two syllables). The repetition gives the poem a rhythmic, sing-song tone that lingers in our memory long after we have read it.