Cosine transformation is NOT LOSSY. After transofmation you just cut some frequences. If you have no information in cut out freequences you get NOT LOSSY compression. So if you cut something once you can't cut it second time. There is no "rounding" or something similar. For example you have sequence of bytes '15 35 156 204 32 0 14 87 5 9 45 34 35 147 21 224' let assume that after cosine transform you have '245 125 54 2 20 6 1 2 0 1 0 0 0 0 0 0'. In order to compress you just cut off last 6 bytes. After decoding you will get exactly the same wav file. So mpeg compressin NOT ALWAYS is lossy. In case of repeatedly compresing-decompresing the same file with the same parameters you will not loose anything.And there is no need to use 192 or even 128 kbps if your sound source is radio quality. You won't get any better qualty. Imagine that instead last 6 zeroes you will cut only 3. You just have bigger files. Here helps much VBR because it will cut more or less depending on how much zeroes you have at the end of spectrum.
Cosine transformation is NOT LOSSY.