Recorded a recent song then saved it and emailed the track to my iphone to see how it sounds through the phone speaker but the vocals almost disappear and sound very distant/reverby (is that a word?). I have sent to other peoples phones and it's the same issue but when the phone is plugged through a hi fi/car stereo the mix sounds fine again. I have also emailed to my pc and again it sounds fine. This has happened before but not on every track I mix and it is only the vocals that almost disappear, everything else sounds as the original mix.
Any help please? 

What are you actually doing?
As you can't send a cwp-file to an iPad or phone, the song is exported first in some form. Which, what, how?
Producing final mixes that transfer well to all soundsystems is the most difficult thing in home recording.

You've got some phase issues with your vocals that are causing them to disappear when you play the song in mono. Try panning the lead vox straight up the middle.

This sounds like you're only getting one channel of the stereo mix playing. 
 
My jack converter (1/4" to 1/8th) sometimes does that when I plug in the cans and that's exactly what it sounds like..... do a mono mix and test that the same way.

I can think of two likely explanations for this phenomenon. The first is that you are using some mono-incompatible widening technique on either the vocal or on the full mix. If, for example, the left and right channels are out of phase when you fold them to mono you'll lose stuff that's center-panned, including vocals. To test if this is your problem, set the interleave on the master bus to mono and listen for extreme differences.
 
The other possibility is that it's just a translation issue. The real trick of mixing is figuring out how to make your song sound acceptable on a wide variety of playback systems, including those at the bottom of the fidelity range.
 
The iPhone is about as lo-fi as it gets. On the one hand, it's incapable of reproducing some frequencies, so any mix elements that depend on those frequencies disappear. But it will also overemphasize other frequencies, exaggerating other mix elements and exacerbating whatever masking effects they're causing. Masking makes things disappear. 

Interesting topic as I am just finishing an album so uploaded a few of the tracks to Sound clicks. 
I told a friend about the album and got him to go to the web page and we listened to few and I was impressed they they actually translated perfectly on his iphone. I was unaware that they are mono too. In my old age I've gotten pretty good at this translating game ,, pat on back if my arm wasn't so sore... 
There's a MONO button on your master buss,,, use it. 

Thanks all, first post so much appreciated. I was simply converting the file to an mp3 so it could be emailed and listened to on other devices to see how it sounds. Don't think I am doing anything tricky with the vocals but will check this and also try switching to mono to see if it solves the problem.
 

Re-reading the OP and the answers I'm also sure it's a phase issue due to mono playback. I just wanted a confirmation that the original exported version is checked to be OK.

pf123
Thanks all, first post so much appreciated. I was simply converting the file to an mp3 so it could be emailed and listened to on other devices to see how it sounds. Don't think I am doing anything tricky with the vocals but will check this and also try switching to mono to see if it solves the problem.
 

Ok, you say you're converting "the file"
 
What file? how did you get your music out of Sonar?

And I didn't mean export in mono, I meant to "check"you mix in mono, Then export in stereo.