Applications access software and hardware features of the phone using a number of APIs. Some features are not directly accessible to applications, either because they lack a suitable API that can be accessed from a Flatpak sandbox, or because they require elevated privileges on the underlying system.
Lower level access to features is described in some detail in the How To Guides for the Librem 5 development kit.
The audio services of the phone can be accessed in a number of ways. These include:
Battery state and charge level can be read and monitored using the
org.freedesktop.UPower D-Bus interface provided by the
upowerd daemon. This is described in the UPower D-Bus API Reference documentation.
Access to information about network connections can be obtained using the
org.freedesktop.NetworkManager D-Bus interface provided by NetworkManager. This is described in the NetworkManager D-Bus APIs documentation.