[RTTY] sound card dynamic range?

[Mark n2qt]

I swapped rigs, moving the FT1000MKV from a Turtle Beach Santa Cruz sound 
card to the
Dell GX260 internal sound card. (the new K3 now has the TB card).   Using 
MMTTY, I
noticed that if I set the audio inputs high enough to decode the water weak 
signals on 15M,
MMTY would complain about Overflow (FT1K using the internal sound card). 
Never saw
this with the TB sound card.  In addition the internal sound card worked 
fine with the previous
Kenwood TS-940.

Since both the TB and internal sound cards are 16 bit devices this confuses 
me.

Any ideas (or do I just need a better sound card).