However, if you fear getting spied on through your smartphone, you'd also have to make sure it can't record ambient sound.
Whether that is paranoid or a possibly used attack vector, is completely up to you and depends on you and the situation you are in.
While I'm quite paranoid about smartphones and cameras, imo if one gets to the point where they're so paranoid that they can't trust their smartphone at all anymore, it may be time to use a dumb phone instead. Like, I do this as a security and privacy practice - not necessarily because I'd be scared of someone monitoring my webcam activity.
For ambient sounds, if you're talking about microphones, it's quite hard to get the 3 microphones disconnected from a smartphone. I don't think there are any (or a lot of) smartphones out there that have modular microphones, they're usually soldered into the motherboard and requires micro-soldering skills to disconnect them. And that implies some very rough possible consequences: you'd have to only use headphones for microphone, so imagine having to call 911 in an emergency.
The thing is, even with your cameras and microphones removed, your phone's hardware and OS are the main issue. If you fear being spied through microphones and cameras, I'd have a much larger fear for the blobs and closed-source stuff the operating system has. At that point, Librem phones should be considered if you really need a smartphone (or a dumb phone, which is as cheap as a meal and can be disposed at any given time).
Another option is to use a Faraday cage to cut all external connections to the phone. Include a white noise generator next to it (or inside the cage) if you're afraid sounds are recorded offline only to be streamed once the phone reconnects to the internet.
