There is no way to guarantee that a person will not be offended when you tell them something like this. You might say the same thing to two different people and have one be offended and one not.
What you can do is try to limit the likelihood and depth of any offence she may take, but whatever you do is almost certainly likely to be awkward for both of you.
I don’t have direct experience of having someone tell me they found my voice annoying, but this is reminding me of a couple of instances where I was made aware that something about my behaviour was irritating a person.
One was at school, I’d not long moved schools and had started hanging about with a certain group at break and lunch, after a little while one member of the group just stopped talking to me. She didn’t avoid me, just kind of pretended I wasn’t there. I was pretty confused until someone told me this girl ‘tired of hearing me talk about my old school’. I’d not realised that I did talk a lot about it and immediately stopped, but she carried on blanking me and I withdrew from that group because the whole thing had become weird and awkward. The girl never told me herself, just letting me find out that people had been talking about me, which made me feel very defensive and wary of interaction.
The other incident was a few years later in my first job. I had developed some sort of nervous habit of closing my eyes when I talked to people. Sounds weird, but I’d be looking at them, face pointed their way with my eyes directed at their face, I’d just have my lids down most of the time. I was vaguely aware of it, but one day someone just asked me not to do it because it was weird to be looked at like that. They weren’t demanding an answer and they then changed the subject and conversation flowed on without giving me a chance to become defensive or for the situation to feel confrontational or combative.
Again I was mortified, but as with the previous incident, now my attention had been brought to the point, it became relatively easy to stop the behaviour. That person never mentioned it again and we carried on as normal, but I was always grateful to know that I’d been being weird and annoying so I could stop.
So, from comparing how those two experienced felt to be on the receiving end of, I suggest you may be most effective and minimise offence by avoiding:
- having a 'serious' conversation about how terrible that voice is *
and instead concentrate on being:
- matter of fact
- move on and treat her as normal
If you tell the person and they change the behaviour, that’s great. If you tell them and they don’t change, well… now you know that they either can’t or won’t change.
Because I was mortified enough to stop the behaviour immediately, it’s harder to recommend strategies that might be effective when someone doesn’t change instantly. But be aware that if people perceive that they are being nagged about something, especially if they can’t see a problem with it themselves, they may be all the more determined to stick to their guns and may even increase the annoying behaviour to spite you.
So my recommendation is a low-key, matter of fact address to her, keep it brief and try to get a compliment in if you think it appropriate, and then carry on the conversation so that you aren’t putting her ‘on the spot’ and demanding a reply or response. perhaps something like:
I do wish you wouldn’t use that voice, your normal voice is so much
nicer and it’s so much easier to work with you when you use it.
Anyway, Dora wants this stuff finished by the end of the day… how will
we split the work between us…
*. I know that I haven't described any experience of being on the receiving end of this kind of approach, but I've included it for contrast to the 'mention and move on' approach. I'm pretty sure that if anyone did do this to me I'd probably go home and cry my eyes out, even if I made a more combative response publicly.