Syncing to iPAQ sometimes gives me blank songs - what gives?

I have Windows Media 10 running on XP, and an HP iPAQ h1945 running the
latest (I think) version of WM mobile.

I'm syncing to a 1GB SD card, and normally have no problem. The songs are a
mix of mp3 and WMA format

Most of my tunes are ripped at 256kbs, so I set WM to convert all syncs to
the card to 128kbs, so I could fit more songs on the card.

Here's what happens - everything converts and syncs as normal, but when I go
to play on the iPAQ, some of the songs are just blank. Silence. The
title/track info is there, but for the duration of the song, there's no
sound. (This isn't a headphone issue or anything like that.) This phenomenon
only happens on songs that have been converted from 256 to 128; if I don't
convert them, they're fine.

Here's the funny part -- there's no detectable pattern in which songs are
silent and which aren't. I'll give an example. I have the album "Sgt
Pepper..." by the Beatles on my iPAQ. All songs ripped from the same CD as
256kbs mp3 files. The song "Sgt Pepper" is completely blank, silent. But
"Lucy in the Sky" and "A Day in the Life" are fine, normal.

Anyone have any idea why this is?