poor sound reproduction using earphones with a 3 section jack plug

Hello

I have found that music, both orchestral and vocal, reproduces almost perfectly when I use earphones that have a 4 section jack-plug, as the ones that were supplied with my iPod touch have. However, the orchestral section is prominant whilst the the vocal section is almost inaudible when I use earphones with a 3 section jack-plug, which I bought recently and were sold marked "for use with iPod touch 4".

To make that clearer -- a 4 section jack plug has three plastic seperator discs on the shaft of the jack plug, whilst a 3 section jack plug has 2 seperators.

Is this normal or is my iPod touch faulty?

Thanks in advance😕