can't recommend any fast n' easy tool since I myself use LAME MP3 Encoder
3.91 and it's frontend RazorLame. But concerning settings, try (if possible
with those tools) to set MP3s to mono mode. Bitrate of about 48 should do
(via the V525's speaker u won't hear any differences to higher bitrates) and
cut the maximum frequences (lowpass) to at least 16000 (might even cut to
13000, gives u extra space for encoded data without real loss of quality,
since as mentioned before on the V525 u won't hear the difference). This way
I get around 370 kb per minute (mono, 48 kbit/s, lowpass: 16000 Hz). If
that's still too large u might reduce bitrate / lowpass further (32 /
10000). But then u'll have to check if it still sounds ok with the V525.

Andrej