Visors with the hasBreather option enabled, but skipping the Radiofication of Shepard's voice in Mass Effect 3, also works for visors in ME 2. Might work for normal helmets as well, but I haven't tested that since I usually only play with the visors.
The modding here requires you to be familiar with appearance modding, which you can learn about over at Aristaea's blog:
http://social.bioware.com/2134634/blog/
What I do here is to make a slight change that will allow you to use visors in vacuum, and toxic locations ( such as Mars, Cyone), but without the need for Radio Voice. That way you don't have to go back to the armory after the mission to get your normal sounding visor.
Only helmets normally have the Breather option, and helmets require a radio to be able to communicate with other people, which is probably why Bioware made it so that the Breather option will always force Shepard's voice into radio mode even if you are technically using a visor.
I added two copies of the same visor, slightly modified versions, but using the exact same ID number. It's a simple trick that makes the game think that we are in fact using a regular visor with the Breather option active, but will skip the radiofication of Shepard's voice.
Make sure you add these two to the Coalesced.bin file in the numbered order: 1 first in the list followed by 2, in the helmetappearances section of the C.bin file
Using here the Sentry Interface as an example:
1. This is the normal version with a couple of small changes:
(Mesh=(Male="biog_hmm_hgr_shp_r.SHPg.HMM_HGR_SHPg_MDL",Female="BIOG_HMF_HGR_SHP_R.SHPg.HMF_HGR_SHPg_MDL"),GameEffects=("SFXGameContent.SFXGameEffect_PartBasedArmor_ShieldBonus"),Id=5,Name=349690,Description=349689,PlotFlag=19007,Type=CustomizableType_Helmet)
2. Second with the added option bHasBreather=True
(Mesh=(Male="biog_hmm_hgr_shp_r.SHPg.HMM_HGR_SHPg_MDL",Female="BIOG_HMF_HGR_SHP_R.SHPg.HMF_HGR_SHPg_MDL",bHasBreather=True),GameEffects=("SFXGameContent.SFXGameEffect_PartBasedArmor_ShieldBonus"),Id=5,Name=349690,Description=349689,PlotFlag=-1,Type=CustomizableType_Helmet)
After adding these to the Coalesced file, you should now find two Sentry Interfaces ( assuming you already found/bought the regular one) in your helmets section at the armory. Pick the first one in the list, so the game will think that it is just the regular visor, and the double ID numbers will do the rest.
Here's the original version of the Sentry Interface, so you can see more clearly how I have changed it:
(Mesh=(MaleVisor="biog_hmm_hgr_shp_r.SHPg.HMM_HGR_SHPg_MDL",FemaleVisor="BIOG_HMF_HGR_SHP_R.SHPg.HMF_HGR_SHPg_MDL"),GameEffects=("SFXGameContent.SFXGameEffect_PartBasedArmor_ShieldBonus"),Id=5,Name=349690,Description=349689,PlotFlag=19007,Type=CustomizableType_Helmet)
