Am now finding that I found a way to get rtl_fm (but not rx_fm) to decode the local classical fm station (using rtl-sdr dongle of course). Still can't get rx_fm (i.e. soapy version) to work. They are using the same code to parse the command lines, so I can change 'rtl_fm' to 'rx_fm' and it should just work, but it does not. It is my understanding that they are supposed to be command line compatible.

Works:
rtl_fm -M wbfm -f 99.5M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -

Doesn't work;
rx_fm -M wbfm -f 99.5M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -

Others should be able to try these two cases just by changing the -f argument to tune to a strong local station.

Version info:

rx_tools$ git log | head -n1
commit 811b21c

Soapy info:

rx_tools$ SoapySDRUtil --info
######################################################

Soapy SDR -- the SDR abstraction library

######################################################

Lib Version: v0.8.0-g926c86d9
API Version: v0.8.0
ABI Version: v0.8
Install root: /usr/local
Search path: /usr/local/lib/SoapySDR/modules0.8
Module found: /usr/local/lib/SoapySDR/modules0.8/libHackRFSupport.so (0.3.3-7d53087)
Module found: /usr/local/lib/SoapySDR/modules0.8/libairspyhfSupport.so (0.1.1-b6cfbf5)
Module found: /usr/local/lib/SoapySDR/modules0.8/libaudioSupport.so (0.1.1-9aa2b5c)
Module found: /usr/local/lib/SoapySDR/modules0.8/libremoteSupport.so (0.6.0-c09b2f1)
Module found: /usr/local/lib/SoapySDR/modules0.8/librtlsdrSupport.so (0.3.1-24b27fa)
Available factories... airspyhf, audio, hackrf, remote, rtlsdr
Available converters...

• CF32 -> [CF32, CS16, CS8, CU16, CU8]
• CS16 -> [CF32, CS16, CS8, CU16, CU8]
• CS32 -> [CS32]
• CS8 -> [CF32, CS16, CS8, CU16, CU8]
• CU16 -> [CF32, CS16, CS8]
• CU8 -> [CF32, CS16, CS8]
• F32 -> [F32, S16, S8, U16, U8]
• S16 -> [F32, S16, S8, U16, U8]
• S32 -> [S32]
• S8 -> [F32, S16, S8, U16, U8]
• U16 -> [F32, S16, S8]
• U8 -> [F32, S16, S8]

Working use case:
rx_tools$ rtl_fm -M wbfm -f 99.5M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -
Found 1 device(s):
0: Realtek, RTL2838UHIDIR, SN: 00000001

Using device 0: Generic RTL2832U OEM

-: (raw)

File Size: 0
Encoding: Signed PCM
Channels: 1 @ 16-bit
Samplerate: 32000Hz
Replaygain: off
Duration: unknown

In:0.00% 00:00:00.00 [00:00:00.00] Out:0 [ | ] Clip:0 Found Rafael Micro R820T tuner
Tuner gain set to automatic.
Tuned to 99771000 Hz.
Oversampling input by: 6x.
Oversampling output by: 1x.
Buffer size: 8.03ms
Exact sample rate is: 1020000.026345 Hz
Sampling at 1020000 S/s.
Output at 170000 Hz.
In:0.00% 00:00:02.30 [00:00:00.00] Out:65.5k [ -===|===- ] Hd:5.6 Clip:0

Failing use case:

rxtools$ rx_fm -M wbfm -f 99.5M | play -r 32k -t raw -e s -b 16 -c 1 -V1 -

-: (raw)

File Size: 0
Encoding: Signed PCM
Channels: 1 @ 16-bit
Samplerate: 32000Hz
Replaygain: off
Duration: unknown

In:0.00% 00:00:00.00 [00:00:00.00] Out:0 [ | ] Clip:0
RtApiAlsa::getDeviceInfo: snd_pcm_open error for device (hw:0,0), Device or resource busy.

RtApiAlsa::getDeviceInfo: snd_pcm_open error for device (hw:0,0), Device or resource busy.

Found Rafael Micro R820T tuner
Using device Audio: device_id=13 origin=https://github.com/pothosware/SoapyAudio
Found 1 channel(s) :
Channel 0 :
Found 1 antenna(s): RX
Found 0 gain(s):
Found 1 frequencies: RF
Found 14 sample rates: 4000 5512 8000 9600 11025 16000 22050 32000 44100 48000 88200 96000 176400 192000
Found 0 bandwidths:
[INFO] Using format CS16.
Bandwidth set to automatic resulted in 0 Hz.
Tuned to 99771000 Hz.
Oversampling input by: 6x.
Oversampling output by: 1x.
Buffer size: 8.03ms
Sampling at 1020000 S/s.
Output at 170000 Hz.

RtApiAlsa::getDeviceInfo: snd_pcm_open error for device (hw:0,0), Device or resource busy.

RtApiAlsa::getDeviceInfo: snd_pcm_open error for device (hw:0,0), Device or resource busy.

RtApiAlsa: dump hardware params just after device open:

ACCESS: RW_INTERLEAVED
FORMAT: U8 S16_LE S16_BE S24_LE S24_BE S32_LE S32_BE FLOAT_LE FLOAT_BE MU_LAW A_LAW S24_3LE S24_3BE
SUBFORMAT: STD
SAMPLE_BITS: [8 32]
FRAME_BITS: [8 1024]
CHANNELS: [1 32]
RATE: [1 192000]
PERIOD_TIME: (5 4294967295)
PERIOD_SIZE: [1 1398102)
PERIOD_BYTES: [128 1398102)
PERIODS: [3 1024]
BUFFER_TIME: (15 4294967295]
BUFFER_SIZE: [3 4194304]
BUFFER_BYTES: [384 4194304]
TICK_TIME: ALL

RtApiAlsa: dump hardware params after installation:

ACCESS: RW_INTERLEAVED
FORMAT: FLOAT_LE
SUBFORMAT: STD
SAMPLE_BITS: 32
FRAME_BITS: 32
CHANNELS: 1
RATE: 192000
PERIOD_TIME: (10666 10667)
PERIOD_SIZE: 2048
PERIOD_BYTES: 8192
PERIODS: 4
BUFFER_TIME: (42666 42667)
BUFFER_SIZE: 8192
BUFFER_BYTES: 32768
TICK_TIME: [0 0]

RtApiAlsa: dump software params after installation:

tstamp_mode: NONE
tstamp_type: GETTIMEOFDAY
period_step: 1
avail_min: 2048
start_threshold: 2048
stop_threshold: -1
silence_threshold: 0
silence_size: 4611686018427387904
boundary: 4611686018427387904
In:0.00% 00:00:01.02 [00:00:00.00] Out:24.6k [ -====|====- ] Clip:0