Paging tones are not created within sound_alsa.c
The audio API is now responsible to switch to paging channel.
In case of SDR, the sdr.c will switch frequency offset.
Sound instance is now called audio instance and uses funcation pointers.
This gives a clean interface to be exchanged with other technologies,
linke SDR.
The last sample may get corrupted. It seems that this depends on the
card that is used. To fix this, the snd_pcm_avail is checked and
one sample less than available is read using snd_pcm_readi. This
sample remains in RX buffer until next read. Then it is not corrupted.
